https://bess.courses/articles/bess-project-key-agreements/ https://bess.courses/figures/agreement-landing.png When each of the project agreements lands, on an axis carrying the article’s stated order rather than elapsed time. First the land option and the interconnection application, and development is mostly these two aging. Then the offtake, signed in the narrow window where the project is de-risked enough to sign but not yet at final investment decision. https://bess.courses/articles/bess-revenue-streams/ https://bess.courses/figures/arbitrage-erosion.png Four bars of arbitrage margin per discharged megawatt-hour for the article’s 100 megawatt, 200 megawatt-hour worked example, each measured against a dashed reference line at the 45 dollar spread on paper. The spread on paper is 45 dollars, an 80 dollar discharge price against a 35 dollar charge price. https://bess.courses/articles/central-vs-string-pcs/ https://bess.courses/figures/pcs-fault-domain.png Two rows comparing what one conversion failure costs a plant. A central power conversion system takes out often a double-digit percentage of plant output — its entire slice goes dark until repair, and the repair is an event with cranes in it. A string or container-level unit takes out a few per cent, and the plant keeps operating; the unit is swap-repairable by a smaller crew. https://bess.courses/articles/sizing-bess-for-ai-data-center-load-ramps/ https://bess.courses/figures/ai-load-timescales.png Five obligations on a data-centre battery placed on a logarithmic time axis running from ten milliseconds to about five hours. An inverter's answer to a phase step sits at 15 milliseconds — the pass criterion in a consultant protocol recommended to ERCOT's inverter-based-resource working group, verified in simulation rather than measured in the field. https://bess.courses/glossary/1500-vdc/ https://bess.courses/figures/1500-vdc.png Four levels from one cell to the class limit. An LFP cell at 3.2 volts nominal inside a protected 2.5 to 3.65 volt window. A module of 104 such cells in series at roughly 330 volts. A string of four of those modules — 416 cells in series — whose working range runs 1,100 to 1,500 volts. https://bess.courses/glossary/ac-block/ https://bess.courses/figures/ac-block.png The plant power path in eight stages. A bracket over the first four — battery racks, DC bus, PCS and unit transformer — marks the AC block as one purchased package, drawn over two ghost copies of itself and labelled one drawing set, one test procedure, N builds. https://bess.courses/glossary/ac-coupled-dc-coupled/ https://bess.courses/figures/ac-coupled-dc-coupled.png AC-coupled versus DC-coupled hybrid plant topologies: on the AC-coupled side the PV with its own inverter and the battery with its own PCS each step up to a shared medium-voltage AC bus; on the DC-coupled side the PV strings and the battery, matched through a DC-DC converter, share one DC bus behind a single inverter and one transformer. https://bess.courses/glossary/amp-hour-capacity/ https://bess.courses/figures/amp-hour-capacity.png Two bars converting the same 314 ampere-hours of charge into energy, one per chemistry, because amp-hours are charge and not energy — the voltage is the exchange rate. At LFP's roughly 3.2 volt nominal, 314 amp-hours books about 1.0 kilowatt-hour, drawn as the reference. At NMC's 3.6 to 3.7 volts the same charge books about 1.13 to 1.16, drawn as an open range because the nominal itself is one. https://bess.courses/glossary/ancillary-services/ https://bess.courses/figures/ancillary-services.png A logarithmic ladder of response times from under a tenth of a second to thirty minutes: a battery converter reaching full power in under 100 milliseconds, then ERCOT fast frequency response at a quarter of a second, GB Dynamic Containment at one second, Continental FCR at thirty, spinning reserve at ten minutes and non-spinning at thirty. https://bess.courses/glossary/augmentation/ https://bess.courses/figures/augmentation.png A capability line starting above a flat contracted reference, fading toward it, and stepping back up twice in tranches. Each step returns capability to at or above the promise, and the promise itself never moves. The gap at day one is the overbuild. https://bess.courses/glossary/authority-having-jurisdiction/ https://bess.courses/figures/authority-having-jurisdiction.png Four offices converging on one approval: the fire marshal or fire code official, the building department, the electrical inspector, and sometimes a state agency or insurer — together granting permission to build and energize, project by project and in writing. https://bess.courses/glossary/automatic-frequency-restoration-reserve/ https://bess.courses/figures/automatic-frequency-restoration-reserve.png Two bars on a time axis running to sixteen minutes, against a dashed reference at the fifteen-minute ceiling on full activation. The PICASSO standard product's five minutes is drawn twice, identically, because the same duration is timed from two different events: from the frequency restoration controller's setpoint under SOGL, and from the connecting TSO's activation request under EBGL. https://bess.courses/glossary/automatic-generation-control/ https://bess.courses/figures/automatic-generation-control.png Two control paths compared. Automatic generation control takes a fresh command from the system operator every two to six seconds in most North American markets — eighteen hundred an hour at a two-second cadence — is tracked at the point of interconnection and scored for performance in the FERC-jurisdictional US markets, and stops when the communications link drops. https://bess.courses/glossary/auxiliary-load/ https://bess.courses/figures/auxiliary-load.png Three bars of station-service draw for one Power Electronics PCSK converter: 0.4 kilowatts with the electronics alone and the ventilation off, 3.1 kilowatts with ventilation on at zero load, and 9.6 kilowatts at full load on four modules. A dashed reference line sits at the 9.6 kilowatt peak — the number the supply has to be sized for — and the gap between each lower bar and that line is hatched. https://bess.courses/glossary/auxiliary-transformer/ https://bess.courses/figures/auxiliary-transformer.png The plant power path with two switches marked on it as cut lines — the DC switch-disconnectors between the DC bus and the PCS, and the medium-voltage switch that an economy mode opens to shed the block transformer’s no-load losses. Below them, four boxes under a heading naming what they are: where the auxiliary transformer’s primary is derived from, and what kills each one. https://bess.courses/glossary/availability/ https://bess.courses/figures/availability.png Three bars of unexcused downtime permitted per year, one per contractual availability guarantee: a 98 per cent guarantee allows about 175 hours, 97 per cent about 263, and 95 per cent about 438 — roughly three weeks. The axis carries a tick at that three-week mark. Two and a half percentage points of guarantee are two and a half weeks of the plant being off the grid. https://bess.courses/glossary/average-state-of-charge/ https://bess.courses/figures/average-state-of-charge.png A state-of-charge axis divided into three regions: below about 30 per cent where charge-direction products park, a 30 to 50 per cent band where idle cells age slowest, and above it the region where discharge-direction reserve and capacity standby park. https://bess.courses/glossary/balance-of-plant/ https://bess.courses/figures/balance-of-plant.png The plant power path with the battery system and the power conversion equipment hatched out at the left, because balance of plant is defined by subtraction, and a second bracket running from the PCS to the HV bay marking scope that is contested — converter skids, MV unit transformers and switchgear, the substation and the gen-tie — which falls inside or outside depending on which contract of the s https://bess.courses/glossary/balance-responsible-party/ https://bess.courses/figures/balance-responsible-party.png One universal obligation with two permitted routes: every market participant, storage operators expressly included, must either be a balance responsible party or contractually delegate that responsibility to one — and either way the deviations land in a BRP's position each settlement period. https://bess.courses/glossary/balancing-capacity/ https://bess.courses/figures/balancing-capacity.png Two contracting lead times three orders of magnitude apart on a logarithmic scale: balancing capacity contracted no more than a day ahead for no more than a day, against capacity-market capacity procured one to four years ahead of delivery. https://bess.courses/glossary/balancing-energy/ https://bess.courses/figures/balancing-energy.png One capacity award splitting into two separate settlements: an availability stream paid per megawatt across the contracted window, and an activated-energy stream paid per megawatt-hour at the platform’s marginal price. Free bids from any prequalified provider enter the second stream without an award. https://bess.courses/glossary/balancing-service-provider/ https://bess.courses/figures/balancing-service-provider.png The seller’s seat drawn as a four-step sequence: prequalification with the connecting transmission system operator, then qualification as a balancing service provider, then bidding — where a capacity award drags mandatory energy bids behind it — and finally delivery on activation. https://bess.courses/glossary/bankability/ https://bess.courses/figures/bankability.png Five evidence streams converging on one verdict: equipment track record for the configuration actually shipping, the guarantee stack and the credit behind it, insurance availability for the chemistry, back-to-back contracts, and an independent engineer's review. https://bess.courses/glossary/battery-energy-supply-agreement/ https://bess.courses/figures/battery-energy-supply-agreement.png Two contract scopes — the battery supply agreement and the balance-of-plant contract — both running into a single node: the owner, who holds every seam between them. Schedule coordination, commissioning handoffs and fault attribution all land there. https://bess.courses/glossary/battery-string/ https://bess.courses/figures/battery-string.png Two marks on a logarithmic scale of fault current, measured in units of a single string’s own contribution. The string itself feeds one unit into a fault, from its own cells through its own contactor. Its fuse must clear roughly a dozen units, because on the order of twelve strings share the common DC bus and a fault inside any one of them is fed by all of the others as well. https://bess.courses/glossary/beginning-of-life/ https://bess.courses/figures/beginning-of-life.png Two bars of capacity: the nameplate measured at the factory after formation at 100 per cent, and beginning of life — what the commercial-operation capacity test measures at the contract boundary — usually around 99 per cent of it. https://bess.courses/glossary/bess-integrator/ https://bess.courses/figures/bess-integrator.png Three warranties drawn as three arrows travelling toward one target, the owner. The system guarantee and the workmanship warranty both arrive. The cell warranty is dashed and ends early against a bar, because it stops at the integrator and never reaches the owner. https://bess.courses/glossary/bid-normalization/ https://bess.courses/figures/bid-normalization.png The same three bids ranked twice. On the left they are ordered by dollars per nameplate kilowatt-hour; on the right by discounted lifecycle cost per delivered megawatt-hour at the point of interconnection. Lines join each bid across the two orderings and cross, because the order changes. Illustrative ordering, not a dataset. https://bess.courses/glossary/busbar/ https://bess.courses/figures/busbar.png Two bars of conductor cross-section for the same continuous current: copper at one times, aluminium at about 1.6 times, reflecting aluminium busbar grades running at roughly 60 to 62 per cent of copper's conductivity. https://bess.courses/glossary/calendar-aging/ https://bess.courses/figures/calendar-aging.png Two calendar-aging curves versus years — a cool cell and a hot cell — both decelerating like the square root of time, the hot one fading about twice as fast. https://bess.courses/glossary/capacity-fade/ https://bess.courses/figures/capacity-fade.png Capacity retention falling from 100 percent of beginning-of-life across fifteen years, with a distinctly steeper first year of roughly 3 percent before settling to about 1.5 to 3 percent a year, reaching an end-of-life threshold of 70 percent. Warranties quote year one separately because it does not behave like the years after it. https://bess.courses/glossary/capacity-maintenance-agreement/ https://bess.courses/figures/capacity-maintenance-agreement.png Five physical preconditions fixed at commercial operation — reserved land and foundations, feeder positions, a compatible DC bus voltage window, PCS and transformer headroom, and code-compliant separation — all required for a later augmentation to be installable at all. https://bess.courses/glossary/capacity-retention/ https://bess.courses/figures/capacity-retention.png A retention number means nothing without its three coordinates — reference, boundary, and test conditions. https://bess.courses/glossary/capacity-warranty/ https://bess.courses/figures/capacity-warranty.png A pre-commercial-operation timeline with three milestones — shipment, energization and commercial operation — each carrying an identical copper caret beneath the axis and joined by the word "or". Each caret marks a place the warranty clock could start, and whichever is chosen decides who owns the calendar fade that accrues in transit and construction storage before it. https://bess.courses/glossary/capex-opex/ https://bess.courses/figures/capex-opex.png Shares of total installed capex, with the two on-top items drawn separately. The battery is 35 to 45 per cent — the line every other decision is argued against. EPC and balance of plant is 30 to 40 per cent, with interconnection site-specific. The power conversion system is 15 to 20 per cent. https://bess.courses/glossary/cell-balancing/ https://bess.courses/figures/cell-balancing.png Three bars of the time a passive balancer needs to bleed off one per cent of state of charge on a 314 amp-hour cell: about 16 hours at 200 milliamps, 31 at 100, and 63 — two and a half days — at 50. One per cent is 3.14 amp-hours, and passive bleed runs at 50 to 200 milliamps per cell, a rate between roughly C/1600 and C/6300. https://bess.courses/glossary/cell-contacting-system/ https://bess.courses/figures/cell-contacting-system.png A cell contacting system: stamped busbars link the terminals of a row of prismatic cells in series, a flexible printed circuit above them taps each junction for voltage sensing, thermistors sit on selected cells, and one harness connector routes all signals to the BMS slave board. https://bess.courses/glossary/cell-imbalance/ https://bess.courses/figures/cell-imbalance.png An LFP open-circuit voltage curve against state of charge: steep at both ends and almost flat between about 8 and 94 per cent, where a plus or minus five millivolt sensing band is drawn to scale and renders as little more than a line across the whole plateau. https://bess.courses/glossary/commercial-operation-date/ https://bess.courses/figures/commercial-operation-date-1.png A project timeline running from mechanical completion through first energization, capacity and performance testing, and permission to operate, to the Commercial Operation Date. Only the final milestone is COD; the four earlier ones are commonly mistaken for it. https://bess.courses/figures/commercial-operation-date-2.png Commercial Operation Date as a gate. On the left, four preconditions must all close: point-of-interconnection energization with permission to operate, a passed capacity and performance test, NFPA 855 closeout with authority-having-jurisdiction sign-off, and independent-engineer or lender acceptance. They converge on the declared COD in the centre. https://bess.courses/glossary/container-enclosure/ https://bess.courses/figures/container-enclosure.png Three generations of container energy in the same roughly twenty-foot footprint: older air-cooled units at one to three megawatt-hours, current liquid-cooled units at about five, and a six-and-a-quarter megawatt-hour class that is shipping — the benchmark peers have since matched or exceeded. https://bess.courses/glossary/contract-energy/ https://bess.courses/figures/contract-energy.png Two contract lines over a project term, with no plant-capability curve on either. One runs flat for the whole term: a fixed guaranteed quantity, held by augmentation. The other steps down in three negotiated steps: a declining guaranteed curve, covered by day-one overbuild. The steps are square, not a slide. https://bess.courses/glossary/control-hierarchy/ https://bess.courses/figures/control-hierarchy.png A five-layer permission stack, system operator at the top and the battery management system at the bottom. Commands descend the left side and any layer may only shrink them; limits ascend the right side, aggregated by worst case and keyed to the limiting cell rather than the average. The bottom layer is marked as the veto, because it holds both the measurement and the contactor. https://bess.courses/glossary/corrective-maintenance/ https://bess.courses/figures/corrective-maintenance.png One failed module forking into three separate costs with three different payers: the part under the equipment warranty, the labour, freight, crane and mobilisation negotiated separately, and the lost revenue recoverable only through capped availability liquidated damages. https://bess.courses/glossary/critical-spare/ https://bess.courses/figures/critical-spare.png Four replacement lead times on a logarithmic scale from days to years, each labelled with the capacity its absence strands: an HVAC compressor available from distributor stock, an out-of-production BMS communications board taking months, an MV switchgear cubicle taking many months, and an HV-class main power transformer at 24 to 36 months or longer with normally no second unit behind it. https://bess.courses/glossary/current-collector/ https://bess.courses/figures/current-collector.png An electrode-potential scale showing where each foil survives: the anode works near 0.1 volts versus lithium, where copper is stable and aluminium would alloy with lithium; the cathode works between 3.2 and 4.2 volts, where aluminium's passivating oxide holds and copper would dissolve. https://bess.courses/glossary/current/ https://bess.courses/figures/current.png Five marks on one logarithmic current scale from 40 amps to 8 kiloamps, tracing the same plant's power through its stages. One 314 amp-hour string at 0.5C carries about 157 amps — a series string carries one cell's current. A 5 megawatt-hour container's DC bus, a dozen strings paralleled, runs near 1,900. https://bess.courses/glossary/curtailment-mitigation/ https://bess.courses/figures/curtailment-mitigation.png Two growth bars for one system in one year: curtailed utility-scale wind and solar rose 29 percent in CAISO during 2024, to 3.4 million megawatt-hours, while the battery fleet on the same system grew 45 percent. Both lines rose. No regulator publishes the figure that would say what one did to the other. https://bess.courses/glossary/cycle-aging/ https://bess.courses/figures/cycle-aging.png Capacity versus cumulative equivalent full cycles: a near-linear decline that knees down as it approaches the warranted cycle count and the end-of-life line. https://bess.courses/glossary/cycle/ https://bess.courses/figures/cycle.png Two bars of identical length, both exactly one equivalent full cycle: a single hundred-per-cent discharge, and two separate swings at fifty per cent depth of discharge. Twice the events book the same single cycle. https://bess.courses/glossary/day-ahead-market/ https://bess.courses/figures/day-ahead-market.png A two-day timeline. On the day before delivery, orders close at twelve noon — a value dated to a July 2024 report that predates the fifteen-minute change. The delivery day that follows is drawn as a bar sliced into ninety-six fifteen-minute market time units, with every fourth division marking an hour boundary, each unit carrying its own price per bidding zone. https://bess.courses/glossary/dc-block/ https://bess.courses/figures/dc-block.png The plant power path with a bracket over its first two stages only — battery racks onto one DC bus — marking the DC block, which ends at a pair of DC terminals. The racks are annotated as one enclosure or several paralleled, because the same name is used at two scales and the one-line diagram decides which a document means. https://bess.courses/glossary/debt-service-coverage-ratio/ https://bess.courses/figures/debt-service-coverage-ratio.png A scale of debt service coverage ratio from 0.9 to 2.2 times cover. Default sits at about 1.0 to 1.05 times, lock-up at about 1.10 to 1.20 times, and a merchant project is sized at about 1.5 to 2.0 times on downside price curves. The gap between where it is sized and where anything breaks is the cushion, and there are two rungs of it. https://bess.courses/glossary/decommissioning-plan/ https://bess.courses/figures/decommissioning-plan-1.png A five-step removal sequence whose first step notes that opening a breaker does not de-energise a rack, and whose discharge step requires the interconnection and O&M contract to still be live — before shipping as Class 9 dangerous goods, removing the high-voltage equipment and restoring the site. https://bess.courses/figures/decommissioning-plan-2.png Two EU recovery targets drawn as scheduled increases rather than fixed figures: recycling efficiency by average weight rising from 65 per cent at the end of 2025 to 70 per cent at the end of 2030, and lithium material recovery rising from 50 per cent at the end of 2027 to 80 per cent at the end of 2031. https://bess.courses/glossary/deflagration-venting/ https://bess.courses/figures/deflagration-venting.png Pressure versus time in a BESS enclosure during a deflagration: the unvented curve blows through the enclosure failure threshold, while the NFPA 68 vented curve opens the vent panel at its static burst pressure and holds the peak at the reduced pressure Pred, below what the structure can survive. https://bess.courses/glossary/degradation-curve/ https://bess.courses/figures/degradation-curve.png Three series sharing one retention chart. A solid copper line lowest is the negotiated warranty floor. A dashed line above it is the expected or P50 model output the financial case runs on. Five sparse dark points sit near the dashed line: the measured capacity tests, and the only measurements on the chart. https://bess.courses/glossary/degradation-knee/ https://bess.courses/figures/degradation-knee.png One retention curve, solid while measurements exist and dashed after they stop, with the end of data marked. Three candidate knee markers sit at visibly different positions along the extrapolated part — one for each detection rule: two-line intersection, maximum curvature, and derivative threshold. All three lie beyond the point where the warranty table stops. https://bess.courses/glossary/degradation/ https://bess.courses/figures/degradation.png Battery capacity retention vs project life in three stages: an initial drop, a long steady linear decline, and an accelerating knee down to the ~70% end-of-life line. https://bess.courses/glossary/deliverable-energy/ https://bess.courses/figures/deliverable-energy.png A band drawn between about 85 and 92 percent of DC nameplate and labelled with those numbers, on an axis reading 70 to 100: deliverable AC at the point of interconnection at beginning of life. https://bess.courses/glossary/delta-connection/ https://bess.courses/figures/delta-connection.png Three windings closed into a triangle, each corner brought out as a line terminal A, B or C. A dashed arrow circulates inside the loop, touching no terminal, and there is no neutral point anywhere on the figure. https://bess.courses/glossary/depth-of-discharge/ https://bess.courses/figures/depth-of-discharge.png A battery pack from 0 to 100 percent state of charge, with the cycled band shaded. Depth of discharge is the SWING across that band on one cycle — how far down from the top the pack is taken — not the band itself and not the reading inside it. https://bess.courses/glossary/derating/ https://bess.courses/figures/derating.png Three bars of continuous capability as a percentage of the rated row, for one 50 degree-rated converter family. Below 40 degrees it delivers 107.7 per cent — more than the plate says. At its rated 50 degrees it delivers 100 per cent. At 60 degrees the bar has no length at all: zero output. A dashed reference line at the datasheet figure runs down the figure, and the shortfall below it is hatched. https://bess.courses/glossary/discount-rate/ https://bess.courses/figures/discount-rate.png Present value of one dollar received in a future year, decaying toward zero, and decaying faster at a higher discount rate. https://bess.courses/glossary/discounted-cash-flow/ https://bess.courses/figures/discounted-cash-flow.png Cumulative discounted cash flow starting negative at the capex outlay, rising with annual net revenue, crossing zero at the discounted payback year, and ending at a positive net present value, with the discounting worked through at 8 percent. https://bess.courses/glossary/division-of-responsibilities/ https://bess.courses/figures/division-of-responsibilities.png A responsibility matrix with six columns — supply, install, terminate, test, witness or approve, and own after commercial operation — over four rows. Three rows are assigned to named parties. The fourth, the SCADA point list and its time base, is left visibly unassigned across every column. https://bess.courses/glossary/duration/ https://bess.courses/figures/duration.png Five bars of duration at rated power, from half an hour to eight, each labelled with the C-rate it implies — 0.5 h is 2C, 1 h is 1C, 2 h is 0.5C, 4 h is 0.25C and 8 h is 0.125C, the C-rate being simply the inverse of the hours. https://bess.courses/glossary/efficiency-fade/ https://bess.courses/figures/efficiency-fade.png Two bars of purchased energy for the same 400 megawatt-hour delivered cycle: about 455 megawatt-hours at 88 per cent net AC round-trip efficiency, and about 465 at 86 per cent after the fade — roughly ten and a half megawatt-hours more, every cycle. https://bess.courses/glossary/electrolyte/ https://bess.courses/figures/electrolyte.png Flash points of the carbonate solvents in a lithium-ion electrolyte, on a temperature scale: dimethyl and ethyl-methyl carbonate flash between about 16 and 25 degrees Celsius — room temperature — while ethylene carbonate flashes above 140. https://bess.courses/glossary/emergency-response-plan/ https://bess.courses/figures/emergency-response-plan.png Five responder actions in sequence: detection, notification and isolation, access, a defend-in-place or controlled-burn strategy, and a re-ignition watch of typically 24 to 72 hours of thermal monitoring after extinguishment. https://bess.courses/glossary/end-of-life/ https://bess.courses/figures/end-of-life.png End of life is a contract line, not a failure — the year the fleet is warranted to still hold its floor. https://bess.courses/glossary/energy-capacity-test/ https://bess.courses/figures/energy-capacity-test.png One measured megawatt-hour result with five stamps that define it: the measurement boundary, the cell temperature, the discharge rate, the state-of-charge endpoints in force, and the date. https://bess.courses/glossary/energy-throughput/ https://bess.courses/figures/energy-throughput.png Three marks four decades apart on a logarithmic scale of cumulative energy: one full cycle of a 400 megawatt-hour plant, about 146,000 megawatt-hours in a year at one cycle a day, and on the order of 2.9 terawatt-hours across a twenty-year term. https://bess.courses/glossary/energy/ https://bess.courses/figures/energy.png A reservoir with an outflow, drawn at the scale of one enclosure: about 5 megawatt-hours installed in a modern outdoor unit, delivered over roughly two hours, so about 2.5 megawatts of flow. Energy is the volume held; power is the rate it can leave; multiplying power by duration is what gives the volume. https://bess.courses/glossary/engineer-of-record/ https://bess.courses/figures/engineer-of-record.png Three discipline-specific engineers of record — civil and structural, electrical, and fire protection — each sealing their own documents, with no single seal covering the whole plant. https://bess.courses/glossary/epc/ https://bess.courses/figures/epc.png Shares of installed capital: the battery system at 35 to 45 per cent — the price that moves fastest — EPC and balance of plant at 30 to 40 per cent with the batteries owner-furnished, and the power conversion system at 15 to 20 per cent. A fourth row sits on top of those three rather than inside them: a high-voltage scope, where the project needs one, adds a further 20 to 25 per cent. https://bess.courses/glossary/equity-capital-contribution-agreement/ https://bess.courses/figures/equity-capital-contribution-agreement.png A commitment early and money late. The equity capital contribution agreement is signed during construction, but nothing moves then. Construction is carried by construction debt and sponsor equity, with the lender sizing its takeout against the committed contribution. https://bess.courses/glossary/equivalent-full-cycle/ https://bess.courses/figures/equivalent-full-cycle.png A scale of equivalent full cycles to about 70 per cent retention, from zero to eleven thousand. Near the origin, a sliver from 300 to 365 marks one year of daily arbitrage. Far along it, two chemistry bands: NMC at roughly 3,000 to 5,000 EFC and LFP at roughly 6,000 to 10,000 or more, both under 25 degree lab conditions. A year of hard cycling is a narrow strip against the warranted count. https://bess.courses/glossary/eu-battery-regulation/ https://bess.courses/figures/eu-battery-regulation.png Two domains separated by a dashed boundary: the product domain, where Regulation 2023/1542 applies identically in every member state, and the installation domain, where national fire and building authorities permit the site under procedures that vary between states. https://bess.courses/glossary/factory-acceptance-test/ https://bess.courses/figures/factory-acceptance-test.png Two component factory acceptance tests, both passed at the works, converging on an integration failure at site — the case where each item conformed and the assembled system did not. https://bess.courses/glossary/failure-rate/ https://bess.courses/figures/failure-rate.png A bathtub curve of failure rate against age. It falls through an infant-mortality region as manufacturing and installation defects burn off, flattens through useful life, and rises again through wear-out. The flat middle is shaded and labelled as the only region where lambda equals one over MTBF and R of t equals exp of minus lambda t. The vertical axis carries no numbers. https://bess.courses/glossary/final-investment-decision/ https://bess.courses/figures/final-investment-decision.png Two lanes on one time axis. On the sponsor lane, the final investment decision: a board or investment committee vote that commits the sponsor and nobody else. Later, on the lender lane, financial close, where conditions precedent are satisfied and the lenders bind. The interval between them is shaded, because it is routinely funded from the sponsor’s own equity. https://bess.courses/glossary/financial-assurance/ https://bess.courses/figures/financial-assurance.png Four instruments compared by what stands behind each and how each behaves when called: a surety bond backed by a surety company that can investigate and raise defences; a standby letter of credit backed by a bank that pays against conforming documents regardless of the dispute; a funded escrow backed by nothing but the cash itself; and a parent guarantee backed by the parent’s balance sheet. https://bess.courses/glossary/financial-close/ https://bess.courses/figures/financial-close.png Five third-party conditions precedent — utility consents, landowner subordination, insurer endorsements, counterparty direct agreements and the independent engineer's report — converging on financial close, the point at which the documents are signed and debt becomes drawable. https://bess.courses/glossary/financing-agreement/ https://bess.courses/figures/financing-agreement.png A strict five-step queue for cash. Operating revenue enters at the top and passes through operating costs, then debt service, then reserve top-ups, before anything reaches equity distributions at the bottom — and those are conditional on the covenant tests. Beside the queue sits construction drawdown, released milestone by milestone against the lenders’ independent engineer’s certificate. https://bess.courses/glossary/four-quadrant-operation/ https://bess.courses/figures/four-quadrant-operation.png A two-by-two grid crossing the real-power choice — charge to the left and discharge to the right, set by dispatch and state of charge — against the reactive choice of injecting or absorbing, set independently by the network's voltage, giving the four quadrants Q1 through Q4. https://bess.courses/glossary/frequency-containment-reserve/ https://bess.courses/figures/frequency-containment-reserve.png Four bars on a linear time axis showing full activation time for frequency containment reserve in four synchronous areas, all measured from the reference incident: Great Britain at 10 seconds, Ireland and Northern Ireland at 15, Continental Europe at 30 and the Nordic area at 30 seconds outside the standard frequency range. https://bess.courses/glossary/frequency-regulation/ https://bess.courses/figures/frequency-regulation.png A stepped line runs left to right about a horizontal zero line, jumping to a new level at every step and crossing the line repeatedly. The area between the line and zero is shaded in the same tone above and below. Above the line is marked discharge and below it charge, and the steps reach about as far below the line as above it. https://bess.courses/glossary/frequency/ https://bess.courses/figures/frequency.png A system frequency scale from 47 to 52 hertz for a 50 hertz synchronous area, with the RfG Continental Europe bands drawn on it: unlimited operation from 49.0 to 51.0, time-limited bands down to 47.5 and up to 51.5, and hard edges beyond. Marks sit at 50 hertz — a 20 millisecond cycle — and at 48.8 hertz, where Great Britain's under-frequency load shedding starts. https://bess.courses/glossary/full-power-deliverability/ https://bess.courses/figures/full-power-deliverability.png Five limits in series, any of which can be the binding one: the battery management system's available power, the DC bus voltage against the converter's current limit, the converter's rating at site conditions and power factor, the transformer and collection system, and the point-of-interconnection cap. https://bess.courses/glossary/fuse/ https://bess.courses/figures/fuse.png A fuse time-current characteristic on log-log axes, from 2 milliseconds to 1000 seconds, against current from 1.2 to 20 times rated current. The lower curve is pre-arcing or melting time; the upper curve is total clearing time; the vertical gap between them at any current is the arcing time, which grows as a proportion as current rises. https://bess.courses/glossary/gearing-leverage/ https://bess.courses/figures/gearing-leverage.png Three returns on one axis. The unlevered project return sits at about 8 percent and the cost of debt at about 6 percent. The equity return at 70:30 gearing is drawn as an open arrow rising from the unlevered return, labelled low teens — the body’s own words, and the only figure it gives. The lift exists only because eight is greater than six. https://bess.courses/glossary/grid-following/ https://bess.courses/figures/grid-following.png A short-circuit-ratio axis at the point of interconnection: above about 3 a grid-following converter is generally stable, between 2 and 3 it is marginal, and below the vendor minimum a project-specific stability study is required. https://bess.courses/glossary/grid-forming/ https://bess.courses/figures/grid-forming.png Grid-forming versus grid-following: a grid-forming inverter behaves as a voltage source that sets its own voltage and frequency, and a given design can respond within a cycle, hold weak grids near short-circuit ratio 1 and black-start — designed, rated capabilities of an implementation, not automatic properties of the mode; a grid-following inverter is a current source tracking the grid through a https://bess.courses/glossary/grounded-wye/ https://bess.courses/figures/grounded-wye.png Three marks in multiples of the line-to-line voltage. About 0.58 before the fault, on any system — the baseline the other two rise from. Then, while one line-to-ground fault is on: about 0.80 on an effectively grounded system, and the full line-to-line voltage on an ungrounded one. https://bess.courses/glossary/grounding-system/ https://bess.courses/figures/grounding-system.png Four voltage levels of one plant, each with a different neutral treatment and a different consequence. https://bess.courses/glossary/hazard-mitigation-analysis/ https://bess.courses/figures/hazard-mitigation-analysis.png A four-step sequence: a bench-scale UL 9540A report feeds a project-specific Hazard Mitigation Analysis, which goes to the authority having jurisdiction for review taking weeks to months per round, and only then to acceptance as a permit condition. https://bess.courses/glossary/hedge-agreement/ https://bess.courses/figures/hedge-agreement.png Two lanes on one unnumbered time axis. The debt tenor runs the full width. Beneath it the hedge tenor stops visibly earlier, and the uncovered remainder is marked in copper as tail years that stay with the project. Three risks never transfer at all: basis between the index and your node, availability shortfalls, and dispatch performance. https://bess.courses/glossary/high-voltage-scope/ https://bess.courses/figures/high-voltage-scope.png The top of a battery plant drawn as five stages: the MV collection bus, the main power transformer, the HV bay carrying breaker, disconnects, arresters and instrument transformers, the gen-tie line, and the point of interconnection. https://bess.courses/glossary/hvac/ https://bess.courses/figures/hvac.png A cell temperature scale from minus 10 to 55 degrees for LFP. Below zero, charging risks lithium plating and racks must be pre-heated first. Zero to 15 is below the window; 15 to 35 is the operating window; 35 to 45 is above it; and above about 45 degrees ageing becomes rapid. https://bess.courses/glossary/i2r-losses/ https://bess.courses/figures/i2r-losses.png Two bars of conduction heat while holding the same megawatts on one 416-cell string. At the roughly 1,330 volt nominal the heat is the baseline — the number every datasheet is quoted at. https://bess.courses/glossary/iec-62619/ https://bess.courses/figures/iec-62619.png A dashed boundary drawn around three nested levels — cell, module and battery system — marking the scope IEC 62619 certifies. The propagation test is applied inside that boundary. One arrow points out of it to three jobs it does not do: container-scale fire spread, the system listing, and siting and separation. https://bess.courses/glossary/iec-62933/ https://bess.courses/figures/iec-62933.png A role-by-role comparison of the US and IEC safety stacks for energy storage. System product safety, test methods and general safety considerations each have an entry on at least one side. The installation row is the exception: the US has NFPA 855, and the IEC column is drawn as a highlighted empty box, because no EU-wide installation code exists. https://bess.courses/glossary/imbalance-settlement/ https://bess.courses/figures/imbalance-settlement.png A two-by-two grid of the four settlement constellations from EBGL Article 55 Table 2, crossing whether the balance responsible party is in surplus or shortage against whether the imbalance price is positive or negative — including the case where a party in surplus pays for it. https://bess.courses/glossary/independent-engineer/ https://bess.courses/figures/independent-engineer.png Payment and reliance separated: the sponsor pays the independent engineer's fee, while the engagement letter names the financing parties as those entitled to rely on the report, and the findings flow to them as struck assumptions, conditions precedent and drawdown certificates. https://bess.courses/glossary/instrument-transformers/ https://bess.courses/figures/instrument-transformers-1.png Four marks on a logarithmic scale of per cent of a current transformer's rated primary current: class 0.2S is guaranteed from 1 per cent, plain class 0.2 only from 5 per cent, and a plant's normal operating point at about 3.3 per cent falls between them — inside one class and below the other. https://bess.courses/figures/instrument-transformers-2.png The same cable run measured as burden on two CT secondaries: about 1.4 volt-amperes on a 1 amp secondary against about 35 on a 5 amp one — twenty-five times more, because burden goes as the square of the secondary current. https://bess.courses/glossary/insurability/ https://bess.courses/figures/insurability.png Three gates in a row. Code compliance and authority-having-jurisdiction approval come first as the regulatory minimum. The carrier’s own engineering review follows and can demand more than code. Terms come third. A dashed arc runs back from the terms to the first gate, because the whole sequence is re-earned at every annual renewal. https://bess.courses/glossary/interconnection/ https://bess.courses/figures/interconnection.png A five-step interconnection sequence: a multi-year queue and study process, then an executed agreement with security posted and upgrade costs assigned, then construction and the physical connection, then model validation and grid-code witness testing, and only then permission to export and import. https://bess.courses/glossary/internal-rate-of-return/ https://bess.courses/figures/internal-rate-of-return.png Two internal-rate-of-return bars whose verdict the labels reverse: a 10 megawatt pilot at 18 per cent, and a 200 megawatt, 800 megawatt-hour plant at 11 per cent that can nonetheless create more total value. https://bess.courses/glossary/internal-resistance/ https://bess.courses/figures/internal-resistance.png Three bars of ohmic heat per cell for a 314 amp-hour LFP cell at an assumed quarter-milliohm. Because P = I²R, heat scales with the SQUARE of C-rate: a quarter-C four-hour duty puts out about 1.5 watts per cell, 0.5C — the source’s worked point — about 6, and 1C about 24. Doubling the current quadruples the heat. https://bess.courses/glossary/intraday-market/ https://bess.courses/figures/intraday-market.png A sequence diagram of the European intraday market. A continuous trading ribbon runs the width of the figure against the shared order book, punctuated by three auction closures: IDA1 at fifteen hundred the day before, IDA2 at twenty-two hundred, and IDA3 at ten in the morning of the delivery day, which allocates only the second half of that day. https://bess.courses/glossary/inverter-topology/ https://bess.courses/figures/inverter-topology.png Top: a two-level leg switches the AC terminal between DC plus and DC minus with four devices per leg, while a three-level leg also reaches the DC mid-point — ten devices per leg for an NPC, eight for a T-type. https://bess.courses/glossary/investment-tax-credit/ https://bess.courses/figures/investment-tax-credit.png A four-rung ladder of investment tax credit as a share of eligible cost: about 6 per cent if prevailing-wage and apprenticeship rules are missed above one megawatt, 30 per cent as the base with them met, and two conditional ten-point adders for domestic content and energy-community siting taking it toward 50, drawn as outlines because they are conditional. https://bess.courses/glossary/iso-rto/ https://bess.courses/figures/iso-rto.png Four clocks of one institution spanning seconds to years on a logarithmic scale: a regulation signal every two to six seconds, real-time energy settled in five-minute intervals, a day-ahead market clearing hourly the day before, and an interconnection queue of roughly three to five years. https://bess.courses/glossary/land-option-agreement/ https://bess.courses/figures/land-option-agreement.png Two lanes starting at the same instant. The option lane runs from signing to financial close. The lease lane starts at exactly the same point, drawn dashed because it is negotiated but not yet in force — rent, escalators, tenor and decommissioning security are already fixed — and then becomes solid at financial close and runs on. https://bess.courses/glossary/lcoe-and-lcos/ https://bess.courses/figures/lcoe-and-lcos.png LCOE and LCOS side by side as fractions. Both divide discounted lifetime cost by discounted lifetime energy. https://bess.courses/glossary/levelized-cost-of-storage/ https://bess.courses/figures/levelized-cost-of-storage.png Two published ranges for the same reference case a year apart. Lazard version 10.0 of June 2025 gives 115 to 254 dollars per megawatt-hour discharged, unsubsidised, for a 100 megawatt four-hour system; version 11.0 of 2026 gives 210 to 292 for the same case. Same metric, same reference case, one year apart. https://bess.courses/glossary/lithium-iron-phosphate/ https://bess.courses/figures/lithium-iron-phosphate.png Two overlapping temperature ranges for the onset of thermal runaway, both marked as cited rather than exact: nickel-rich NMC from about 150 to 210 degrees Celsius, and LFP from about 200 to 270. The two ranges overlap between 200 and 210 degrees. https://bess.courses/glossary/long-term-service-agreement/ https://bess.courses/figures/long-term-service-agreement.png Two spans starting together at commercial operation. The battery supply agreement’s workmanship warranty is short. The long-term service agreement runs far longer and carries the numbers that matter — capacity retention by year, round-trip efficiency and availability — but only for as long as it stays alive. https://bess.courses/glossary/lower-explosive-limit/ https://bess.courses/figures/lower-explosive-limit.png A scale of hydrogen concentration in air from 0 to 8 percent by volume. The lower explosive limit sits at 4 percent, and everything above it to 75 percent is flammable. The NFPA 69 design target is to stay below 1 percent — a quarter of the LEL — and a typical warning alarm fires at about 0.4 percent, a tenth of it. https://bess.courses/glossary/main-power-transformer/ https://bess.courses/figures/main-power-transformer.png Three bars from one transformer nameplate, one per cooling stage: ONAN at 75 MVA, ONAF at 100 MVA and OFAF at 125 MVA, on an illustrative large unit. A dashed reference line sits at the ONAN rating, the only stage that needs nothing running to hold it; the two longer bars are drawn above it and labelled as holding only while the fans, or the fans and pumps, are running on station service. https://bess.courses/glossary/manual-frequency-restoration-reserve/ https://bess.courses/figures/manual-frequency-restoration-reserve.png Three bars on a time axis running to twenty-nine minutes. The first spans zero to fifteen minutes: the activation interval, anywhere inside which a direct instruction can land. The second runs from zero to 12.5 minutes, the MARI standard product's full activation time for an instruction scheduled at the start. https://bess.courses/glossary/mean-time-between-failures/ https://bess.courses/figures/mean-time-between-failures.png Two bars contrasting a misreading with a meaning: dividing a 500,000-hour mean time between failures by the hours in a year gives an apparent 57-year life, while what the figure actually says is that a fleet of a hundred units running for a year expects between one and two failures among them. https://bess.courses/glossary/mean-time-to-repair/ https://bess.courses/figures/mean-time-to-repair.png The six segments of a restoration drawn left to right — detection, diagnosis, mobilisation, logistics delay, physical repair and return to service including re-test. Two brackets marked contractual MTTR cover the first three segments and the last two, with a hatched bracket over the fourth: logistics delay, commonly carved out of contractual MTTR and tracked separately as mean logistics delay time. https://bess.courses/glossary/measurement-boundary/ https://bess.courses/figures/measurement-boundary.png The plant power path with five vertical cut lines on it — at the cell terminals, the DC bus at the rack, the PCS AC terminals, the unit transformer's MV terminals, and the POI, the settlement boundary — and, headed on the figure as one quantity, LFP round-trip efficiency quoted at three of them: about 92 to 96 per cent at the DC bus, 88 to 93 per cent at the PCS AC terminals, and 85 to 90 per cent https://bess.courses/glossary/mechanical-completion/ https://bess.courses/figures/mechanical-completion.png A sequence of per-block mechanical-completion certificates rather than a single date, followed by the commissioning and capacity-testing period that commonly runs two to four months on a hundred-megawatt-class plant. https://bess.courses/glossary/merchant-vs-contracted/ https://bess.courses/figures/merchant-vs-contracted.png Two required minimum debt-service coverage ratios: roughly 1.20 to 1.40 times for a contracted project sized against its fee, and roughly 1.5 to 2.0 times and upward for a merchant one sized against a P90 downside case. https://bess.courses/glossary/mv-collection-system/ https://bess.courses/figures/mv-collection-system.png A radial 34.5 kV feeder drawn as five boxes standing for consecutive cable segments, running left to right from the far end to the collection bus, numbered segment 1 through segment 5 in the direction the current grows, with a 4.4 MVA block tapped onto each from below. https://bess.courses/glossary/mv-lv-hv/ https://bess.courses/figures/mv-lv-hv.png Three AC voltage classes across one plant, drawn as rising steps: low voltage at the cells and PCS below 1.5 kV DC and 400 to 800 V AC, medium voltage for array collection at 10 to 35 kV, and high voltage at the point of interconnection at 60 to 345 kV, with a transformer between each pair. https://bess.courses/glossary/mv-switchgear/ https://bess.courses/figures/mv-switchgear.png Five marks on one logarithmic current scale spanning about 50 amps to 90 kiloamps, each family plotted at its lowest member. Near the left, close together: one 4,200 kVA power block draws about 70 amps at 34.5 kV and about 176 at 13.8 kV, and the cubicle's continuous rating is 400 or 630 amps. https://bess.courses/glossary/mva-headroom/ https://bess.courses/figures/mva-headroom.png Five bars of the reactive headroom left at each real-power loading on the apparent-power circle, from the identity Q over S equals the square root of one minus P over S squared. At 80 per cent real-power loading, 0.60 per unit of reactive headroom remains; at 90 per cent, 0.44; at 95, 0.31; at 99, 0.14; and at 100 per cent, none. https://bess.courses/glossary/mva/ https://bess.courses/figures/mva.png Two bars of apparent power for one converter carrying the same 3,680 amps: 4.4 megavolt-amperes at 690 volts, and about 3.96 megavolt-amperes at 0.90 per-unit voltage — about ten per cent less for identical current. https://bess.courses/glossary/nameplate/ https://bess.courses/figures/nameplate.png The plant power path with three cut lines on it, one per nameplate, because a project carries three and they are in three different units. At the battery the nameplate is DC energy in megawatt-hours. At the PCS and the transformer it is apparent power in megavolt-amperes — and a 100 MVA converter at 0.95 power factor can deliver at most 95 megawatts of real power. https://bess.courses/glossary/nationally-recognized-testing-laboratory/ https://bess.courses/figures/nationally-recognized-testing-laboratory.png Three steps: OSHA grants recognition per test standard and per testing site, the recognized laboratory then tests, lists and surveils under its own registered mark, and the result satisfies the code's listing requirement equally whichever lab issued it. https://bess.courses/glossary/negative-sequence/ https://bess.courses/figures/negative-sequence.png Three sets of three phasors drawn as arrows from a common origin. The middle one is emphasised: the negative-sequence set, three equal arrows 120 degrees apart but turning in reversed a-c-b order, against the positive-sequence set turning a-b-c and the zero-sequence set with no displacement and no rotation. https://bess.courses/glossary/net-present-value/ https://bess.courses/figures/net-present-value.png Cumulative discounted cash flow over a project life: a negative capex outlay at year zero, then discounted annual inflows climbing back through the axis. Net present value is where the curve ENDS — its final height above or below zero. https://bess.courses/glossary/neutral-current/ https://bess.courses/figures/neutral-current.png Five bars of neutral current as a percentage of one phase's fundamental, against a dashed reference at the phase current itself. A balanced four-wire wye with no triplen content carries nothing at all — drawn as a bar of no length, because three equal currents 120 degrees apart sum to zero at every instant. https://bess.courses/glossary/nfpa-855/ https://bess.courses/figures/nfpa-855.png Two marks on a logarithmic scale of lithium-ion energy per fire area: the 600 kilowatt-hour figure from the 2020 and 2023 editions, and a single current liquid-cooled container at about 5,000 kilowatt-hours — roughly eight times over it. https://bess.courses/glossary/nmc/ https://bess.courses/figures/nmc.png Two cycle-life ranges on one scale: NMC from about 3,000 to 5,000 cycles to 80 per cent state of health, and LFP from about 6,000 to more than 10,000. Each row also carries its cell energy density — about 200 to 260 watt-hours per kilogram for NMC against about 160 to 190 for LFP. https://bess.courses/glossary/nominal-energy/ https://bess.courses/figures/nominal-energy.png Three rungs of the energy ladder as a percentage of the nominal figure it starts from. Nominal is pure arithmetic — amp-hour capacity times nominal cell voltage times cell count — and is drawn as a solid bar at 100 per cent, because no meter ever reads it. https://bess.courses/glossary/nominal-voltage/ https://bess.courses/figures/nominal-voltage.png Three readings on one 416-cell-series LFP string, on a voltage axis. Conductors and protection are sized at the low-SOC floor, 900 to 1,040 volts. The nominal sits at about 1,331 volts — 416 cells at 3.2 volts each, with 1,150 to 1,330 volts covering the spread of series counts other vendors use — and nothing at all is checked there; it is the label and only the label. https://bess.courses/glossary/notice-to-proceed/ https://bess.courses/figures/notice-to-proceed.png One dated notice with four consequences hanging from it: the time for completion runs from it, the guaranteed substantial-completion and commercial-operation dates are computed from it, the milestone payment ladder is indexed to it, and the delay-damages baseline is measured from it. https://bess.courses/glossary/o-and-m-agreement/ https://bess.courses/figures/o-and-m-agreement.png Two maintenance scopes separated by a boundary: the long-term service agreement covering the battery OEM's cells, enclosures and controls, and the operations and maintenance agreement covering the balance of plant — switchgear, transformers, SCADA, site systems, vegetation, security and first response. https://bess.courses/glossary/off-gassing/ https://bess.courses/figures/off-gassing.png Three stages of a cell failure in sequence: the cell overheats and vents, then releases a largely invisible hydrogen and carbon-monoxide-rich plume that parts-per-million gas sensors can detect, and only later produces the smoke and flame that smoke and heat detectors respond to. https://bess.courses/glossary/offtake-agreement/ https://bess.courses/figures/offtake-agreement.png Offtake structures ordered by who carries market risk, against three commercial traits: a tolling agreement removes market risk entirely but trades upside away for the fee; a capacity or resource-adequacy contract puts a floor under revenue with upside left on the uncontracted part; contract-for-difference, floor and hedge structures leave dispatch with the project and only cap the downside; and a https://bess.courses/glossary/ohm-s-law/ https://bess.courses/figures/ohm-s-law.png Ohm’s law drawn as a symbolic relationship: voltage equals current times resistance, with the two working directions — multiplying current through milliohm paths gives the sag and cable drop that press on the PCS window, and dividing a stiff source voltage by a deliberately tiny resistance gives the kiloamp scale of a DC fault. https://bess.courses/glossary/open-circuit-voltage/ https://bess.courses/figures/open-circuit-voltage.png A per-cell open-circuit voltage scale for LFP from 2.4 to 3.8 volts, measured at zero current after full relaxation. Between the 2.5 volt floor and about 2.9 the curve is steep and voltage genuinely reads state of charge. From about 2.9 to 3.45 is the plateau. https://bess.courses/glossary/operating-window/ https://bess.courses/figures/operating-window.png Three nested ranges on an axis carrying no numbers, labelled voltage, current, state of charge and temperature — the same three layers on each. The outermost is the absolute limits, beyond which protection trips. Inside it sits the continuous window, where the equipment stays connected but derated. Innermost is the full-rating window, the only place the datasheet headline exists. https://bess.courses/glossary/overbuild/ https://bess.courses/figures/overbuild.png Four bars grossing up from the number a contract actually asks for to the DC nameplate that delivers it. Starting at 400 MWh of usable AC at the point of interconnection in year twenty, net of round-trip efficiency and auxiliaries: dividing by 0.70 end-of-life retention gives about 571, then by the 0.90 SOC window about 635, then by 0.95 one-way conversion about 670 MWh of DC nameplate. https://bess.courses/glossary/owner-s-engineer/ https://bess.courses/figures/owner-s-engineer.png The same finding placed at two points in a project's life: a warranty-envelope gap caught during bid evaluation, which costs a negotiation, against the identical clause discovered in year eight, which costs a claim. https://bess.courses/glossary/p-f-droop/ https://bess.courses/figures/p-f-droop.png Three bars showing what one 0.1 hertz dip commands from a 100 megawatt plant under the droop law. At 4 per cent droop on a 60 hertz system, about 4.2 megawatts — full rated swing spread over 2.4 hertz. At the same 4 per cent on a 50 hertz system, 5 megawatts, because the same percentage of a tighter nominal is more megawatts per hertz. At 2 per cent droop, 10: halving the droop doubles the answer. https://bess.courses/glossary/p-q-capability-test/ https://bess.courses/figures/p-q-capability-test.png Two rows share one axis that runs for as long as a single commanded point is held. On the first, a filled dot marks the moment the commanded real and reactive point is reached, a few seconds in. On the second, a bar runs the rest of the width: the point held while converter, transformer and enclosure temperatures rise over minutes, with whether it can be sustained still an open question. https://bess.courses/glossary/p-q-capability/ https://bess.courses/figures/p-q-capability.png Battery four-quadrant P-Q capability circle with a worked example: a 100 MVA converter at power factor 0.95 delivers P = 95 MW and Q = 31 MVAr, and S equals the square root of P squared plus Q squared, which sits on the edge of the circle. https://bess.courses/glossary/p50-p90/ https://bess.courses/figures/p50-p90.png A scale of annual revenue as a percentage of the P50 case, from 55 to 105 per cent. A wide shaded band from 60 to 85 per cent marks where a merchant project's P90 commonly falls, 15 to 40 per cent below P50, and it is labelled as the band debt is sized inside — anywhere in it, not at a point. A narrow band from 96 to 100 marks a fully tolled project's P90, only a few per cent below. https://bess.courses/glossary/parallel-connection/ https://bess.courses/figures/parallel-connection.png Two bars of amp-hour capacity: one 416-cell rack at 314 amp-hours, and two of the same racks in parallel at 628 — while the voltage label on both rows stays at about 1,330 volts, unchanged. https://bess.courses/glossary/payback-period/ https://bess.courses/figures/payback-period.png Cumulative discounted cash flow over a project life: a negative capex outlay at year zero, then discounted annual inflows climbing back. Payback is where the curve CROSSES zero — the year the outlay is recovered, saying nothing about what happens after. https://bess.courses/glossary/peak-shaving/ https://bess.courses/figures/peak-shaving.png A four-step statutory sequence for the EU peak-shaving product: a Council-declared price crisis, then procurement no more than a week before activation, then activation before or within the day-ahead timeframe, then control against a consumption baseline. https://bess.courses/glossary/per-unit-system/ https://bess.courses/figures/per-unit-system-1.png One transformer's impedance marked twice on a logarithmic per-unit scale: 0.07 per unit on its own 5 megavolt-ampere nameplate base, and 1.40 per unit on a 100 megavolt-ampere study base — the same tank, a factor of twenty apart. https://bess.courses/figures/per-unit-system-2.png One label, 1.0 per unit, forking into the three different quantities it names depending on where it is read: the nameplate apparent power on a capability chart, the converter's own rated current on a fault-contribution line, and nominal voltage at a bus. https://bess.courses/glossary/performance-guarantee/ https://bess.courses/figures/performance-guarantee.png Five links in sequence that together make a performance guarantee enforceable: the measurement boundary, the named test method, the operating conditions, the exclusions list, and the remedy with its cap. https://bess.courses/glossary/performance-normalization/ https://bess.courses/figures/performance-normalization.png One comparison reachable by two legitimate routes: correcting the measured result to the guarantee's reference conditions, or redrawing the guarantee at the conditions of the test day. https://bess.courses/glossary/phase-angle/ https://bess.courses/figures/phase-angle.png Two panels, each holding two arrows. In the first, a voltage arrow and a current arrow at one point are separated by the angle phi, which sets the power factor. In the second, the voltages at two different buses are separated by the small angle delta, across which real power flows. https://bess.courses/glossary/phase-sequence/ https://bess.courses/figures/phase-sequence.png Two sets of three phase arrows at identical magnitudes. The first turns in the order A, B, C; the second, drawn the same size, turns A, C, B. Of the six ways to write three letters the cyclic rotations coincide, so only these two orders exist. https://bess.courses/glossary/phase-unbalance/ https://bess.courses/figures/phase-unbalance.png A voltage unbalance scale from zero to six per cent, on the IEC basis — negative-sequence over positive-sequence. EN 50160 permits zero to two per cent for 95 per cent of weekly ten-minute means, with up to about three per cent allowed where single-phase-connected installations dominate; beyond three is outside the standard. https://bess.courses/glossary/phasor/ https://bess.courses/figures/phasor.png One cycle of a sinusoid beside the single arrow it reduces to, both drawn at the same scale: the wave rises to a peak of about 563 volts, a dashed level across it marks the 398 volt RMS value, and the arrow beside it reaches only that shorter RMS level — not the peak — at an angle measured from a marked zero-degree reference axis. https://bess.courses/glossary/planned-outage/ https://bess.courses/figures/planned-outage.png Two mechanics for excluding planned outage hours from an availability calculation, side by side: removing them from the denominator, or deeming the plant available in the numerator — the same window computed two ways. https://bess.courses/glossary/poi-capability-envelope/ https://bess.courses/figures/poi-capability-envelope.png A real-power against reactive-power plane, both axes unnumbered and drawn to one common scale. Three thin curves nest inside one another, sharing the same flat top and bottom: the same converter’s capability at the inverter terminals at three ambient temperatures, with the band between the outer and inner curve hatched to show what the ambient rows alone cost with no change to the hardware. https://bess.courses/glossary/positive-sequence/ https://bess.courses/figures/positive-sequence.png Three sets of three phasors drawn as arrows from a common origin. The first, emphasised, is the positive-sequence set: three equal arrows 120 degrees apart turning in normal a-b-c order. Beside it, the negative-sequence set, equal and 120 degrees apart but turning a-c-b, and the zero-sequence set, three equal arrows with no displacement between them and no rotation at all. https://bess.courses/glossary/power-fade/ https://bess.courses/figures/power-fade.png Available power against state of charge, with no numbers on either axis. A flat converter ceiling runs the full width and does not move with age. A solid battery envelope rises above it through the mid-range and falls to zero at both extremes of state of charge. A dashed aged envelope sits strictly inside it and crosses back under the ceiling much earlier at both ends. https://bess.courses/glossary/power-purchase-agreement/ https://bess.courses/figures/power-purchase-agreement.png Three contracted numbers, each wired by an arrow to the design decision the body says it drives: the megawatt and megawatt-hour figures at the point of interconnection drive DC oversizing and planned augmentation; the availability and response-time commitments drive redundancy in the converter and balance of plant; and the capacity-retention schedule drives warranty and long-term-service alignment https://bess.courses/glossary/power/ https://bess.courses/figures/power.png Power read as a rate against the energy behind it: a 100 megawatt plant drawn with 400 megawatt-hours of storage, a four-hour system. The same power rating paired with a quarter of that energy is a one-hour system chasing different revenue, because power and energy are sized independently. https://bess.courses/glossary/pre-cod-degradation/ https://bess.courses/figures/pre-cod-degradation.png A scale of capacity lost between cell manufacture and commercial operation, from zero to six per cent, for LFP and calendar-only — nothing here is caused by cycling. Roughly 2 to 4 per cent is ordinary first-year calendar fade; below that means a fast programme, and above it means a hot laydown yard or a slipped energisation date. https://bess.courses/glossary/predictive-maintenance/ https://bess.courses/figures/predictive-maintenance.png Two domains separated by a boundary: the slow, population-level trends that condition monitoring genuinely predicts, and the sudden internal short from a latent defect that it cannot, developing on a timescale no historian resolves. https://bess.courses/glossary/prequalification/ https://bess.courses/figures/prequalification.png A statutory sequence from application through an eight-week completeness check and a three-month evaluation to qualification, which is granted per product, and then re-assessment at least every five years or on equipment change. https://bess.courses/glossary/preventive-maintenance/ https://bess.courses/figures/preventive-maintenance.png A four-step cascade in which an overdue filter lets a cooling loop lose capability without tripping anything, cells then run a few degrees warm through a hot season, and the cost appears on the warranted retention curve rather than in the outage log. https://bess.courses/glossary/propagation/ https://bess.courses/figures/propagation.png Four scales in sequence — cell, module, rack and container — each paired with the barrier meant to stop propagation there. The UL 9540A cell-level test characterises a single cell's runaway and its gases rather than propagation; whether runaway spreads from cell to cell is what the module-level test forces and measures. https://bess.courses/glossary/protection-relay/ https://bess.courses/figures/protection-relay.png A settings axis squeezed from both ends. The lower bound is coordination — the element must clear its own zone and grade above downstream devices. The upper bound is compliance — it must not trip inside the ride-through envelope that NERC PRC-024, superseded for inverter-based resources by PRC-029, imposes as a constraint on settings. Between them lies the permitted corridor. https://bess.courses/glossary/q-v-droop/ https://bess.courses/figures/q-v-droop.png A bus voltage scale from 90 to 109 per cent of nominal carrying California Rule 21's default volt-var curve as zones. Between 97 and 103 per cent is the deadband, at unity — no reactive exchange. From 97 down to 92 the plant injects reactive power, reaching 30 per cent of rating at 92; from 103 up to 107 it absorbs, reaching 30 per cent at 107. https://bess.courses/glossary/ready-to-build/ https://bess.courses/figures/ready-to-build.png Four secured checklist items marked on a sequence line — site control, permits in force, an executed interconnection agreement and a defined connection scope. Copper frayed tails run off the first three, because each of them expires. https://bess.courses/glossary/reliability-run/ https://bess.courses/figures/reliability-run.png The same 97 per cent availability figure measured over two windows: a fourteen-day reliability run allows about ten hours of unexcused downtime in total, while an operating year allows about 263. https://bess.courses/glossary/reliability/ https://bess.courses/figures/reliability.png An exponential survival curve falling from 100 percent of units. At one MTBF it crosses about 37 percent, not 50, so most of the population has already failed once by that point. A dashed grey line marks the 50 percent half-life the intuition assumes and the curve never grants there. https://bess.courses/glossary/remote-terminal-unit/ https://bess.courses/figures/remote-terminal-unit.png Two ends — the plant side presenting points, and the control-centre side polling them — both running into one shared artefact: the revision-controlled point map, carrying index, type, meaning, units, scale, deadband and direction for every row. https://bess.courses/glossary/request-for-proposal/ https://bess.courses/figures/request-for-proposal.png A comparison grid whose two columns carry the identical headline — 100 megawatts and 400 megawatt-hours, both compliant — over five rows where the bids quietly differ: the year the energy is guaranteed, the bus it is guaranteed at, whether augmentation is inside the price, how availability is counted, and whether long-term service is attached. https://bess.courses/glossary/resistance-growth/ https://bess.courses/figures/resistance-growth.png Four bars of ohmic heat for one 5 MWh-class container cycling at 0.5C, against a dashed reference at its beginning-of-life figure of about 30 kilowatts. https://bess.courses/glossary/resistance/ https://bess.courses/figures/resistance.png Three marks on a logarithmic resistance scale, one per element of the series chain a battery's current actually crosses. One laser weld is tens of microohms — and there are hundreds per module. A cell's DC internal resistance is a few tenths of a milliohm. A DC collection run is fractions of a milliohm, at about 0.06 milliohms per metre for 300 square-millimetre copper. https://bess.courses/glossary/revenue-meter/ https://bess.courses/figures/revenue-meter.png Two error magnitudes: the revenue meter's own accuracy class of plus or minus 0.2 per cent, and a single phase-displacement term from the instrument transformers feeding it, worth about 0.5 per cent at ten arcminutes and a power factor of 0.5 — wider than the meter's whole class band. https://bess.courses/glossary/ride-through/ https://bess.courses/figures/ride-through-1.png The ride-through voltage envelope: the LVRT floor holds near zero for 150 milliseconds then ramps back to 0.9 per unit, the HVRT ceiling allows 1.2 per unit for up to a second then 1.1 continuously, and the plant must stay connected anywhere between the two boundaries. https://bess.courses/figures/ride-through-2.png What the current does inside the ride-through envelope, drawn symbolically: the reactive current ordered equals the code’s gain times the voltage deviation, all inside one converter current limit — so a shallow sag leaves room to keep exporting, and a deep sag hands the whole limit to voltage support and real power goes to zero by design. https://bess.courses/glossary/root-cause-analysis/ https://bess.courses/figures/root-cause-analysis.png A causal sequence read from its root: an overdue filter or a setpoint permitting an unsized duty leads to a failed cooling fan, which leads to the trip on high cell temperature that is the only thing actually observed. https://bess.courses/glossary/root-mean-square/ https://bess.courses/figures/root-mean-square.png Three pairs of marks on one logarithmic voltage scale, each joining an RMS label to the peak the same waveform actually reaches: 230 volts to about 325, 400 volts to about 566, and 690 volts to about 976. Because the ratio is the same 1.414 every time, the three hops are the same length. https://bess.courses/glossary/round-trip-efficiency/ https://bess.courses/figures/round-trip-efficiency.png A 100 per cent starting point walked down through 4 losses to 90.3 per cent. https://bess.courses/glossary/scada/ https://bess.courses/figures/scada.png One detected event splitting into two paths: a local hard-wired path where the battery management system opens the contactor and protection and fire systems act in milliseconds without any network, and a supervisory path where SCADA annunciates, logs and historizes on a one-to-four-second cycle. https://bess.courses/glossary/series-connection/ https://bess.courses/figures/series-connection.png Voltage as a series chain gets longer, on a logarithmic axis. One LFP cell at about 3.2 volts nominal, 314 amp-hours. One module of 104 cells in series — 1P104S — at about 330 volts, still 314 amp-hours. One string of four such modules, 416 cells in series, working across 1,100 to 1,500 volts, still 314 amp-hours. https://bess.courses/glossary/setpoint/ https://bess.courses/figures/setpoint.png A control loop: a setpoint enters a comparator, the controller drives the BESS, the measured output feeds back and is subtracted from the setpoint to form the error the controller chases. https://bess.courses/glossary/sign-convention/ https://bess.courses/figures/sign-convention.png One power path with three boundaries marked on it, each carrying a different meaning for the same positive sign: at the battery management registers a positive number means charging, at the grid-facing converter and SCADA it means export, and at the revenue meter it may be configured to count delivery to the plant as positive instead. https://bess.courses/glossary/single-phase-vs-three-phase/ https://bess.courses/figures/single-phase-vs-three-phase.png Top: three sine waves displaced by 120 degrees. Bottom: instantaneous power, drawn as three stacked bands. At unity power factor one phase alone follows a sine-squared curve that reaches zero twice per cycle and never goes negative; the three balanced phases stacked together reach a flat line at 1.5 times the single-phase peak, with the single-phase peak marked at 1 for comparison. https://bess.courses/glossary/site-acceptance-test/ https://bess.courses/figures/site-acceptance-test.png Two tests asking two different questions. The factory acceptance test asks whether the item conforms to its specification. The site acceptance test asks whether the installation conforms to standards, codes, specifications and installation instructions — and it cannot start until all equipment is installed and interconnected with permission to exchange power, a precondition the works cannot supply. https://bess.courses/glossary/soc-window/ https://bess.courses/figures/soc-window.png A battery pack from 0 to 100 percent state of charge. Only the band from 2 to 98 percent is cycled; the 2 percent above it and the 2 percent below it are reserved. Usable energy equals depth of discharge times nameplate. https://bess.courses/glossary/sodium-ion/ https://bess.courses/figures/sodium-ion.png Two bars of container energy: an LFP-based container at 100 per cent, and a sodium-ion container in the same enclosure holding 60 to 80 per cent of it — 20 to 40 per cent fewer megawatt-hours for the same box. https://bess.courses/glossary/solid-electrolyte-interphase/ https://bess.courses/figures/solid-electrolyte-interphase.png Two bars of cyclable lithium inventory: 100 per cent before formation, and 90 to 95 per cent after the first charge cycles — 5 to 10 per cent of the lithium is consumed building the passivation film itself. https://bess.courses/glossary/special-purpose-vehicle/ https://bess.courses/figures/special-purpose-vehicle.png Five project assets and contracts — the plant, the interconnection agreement, the EPC and supply contracts, the land lease and easements, and the offtake with the safety evidence file — all held inside a single special purpose vehicle that lenders deal with on a non-recourse basis. https://bess.courses/glossary/state-of-charge/ https://bess.courses/figures/state-of-charge.png A battery pack from 0 to 100 percent state of charge with the permitted band from 2 to 98 percent drawn on it, and a single heavy line across it at 50 percent: the reading right now. A shaded margin either side of that line marks the roughly 3 percent the estimator can be out by between calibrations. https://bess.courses/glossary/state-of-health/ https://bess.courses/figures/state-of-health.png State of health falling from 100 percent of rated beginning-of-life nameplate across fourteen years at roughly 1 to 3 percent a year, tapering, to the 70 percent warranty endpoint. The endpoint is a contractual convention rather than a physical cliff, and capacity is only one of the measures the term covers. https://bess.courses/glossary/substantial-completion/ https://bess.courses/figures/substantial-completion.png One milestone with three consequences branching from it: it transfers care, custody, control and risk of loss to the owner; it starts the workmanship warranty and defects-liability period; and it stops delay liquidated damages, which price lateness. https://bess.courses/glossary/technical-due-diligence/ https://bess.courses/figures/technical-due-diligence.png Four pairs of documents, each pair complete on both sides and contradicting itself across a fracture mark: the offtake and the warranty guarantee energy at different buses; the tolling agreement grants more cycling than the warranty envelope permits; the fire-test report covers a cell revision the factory no longer ships; and the revenue model dispatches a duty the guarantee was never written for. https://bess.courses/glossary/telemetry/ https://bess.courses/figures/telemetry.png Two data paths side by side. Telemetry carries measurement and status — active and reactive power, voltage, frequency, statuses, state of charge and the live charge and discharge limits — specified by point list, cadence, protocol and path availability. The revenue meter carries settlement, specified by accuracy class. State of charge appears only on the telemetry path. https://bess.courses/glossary/thermal-runaway/ https://bess.courses/figures/thermal-runaway.png A scale of cell temperature from 0 to 500 degrees Celsius for LFP. Normal operation ends around 55 degrees, where the battery management system derates and then trips. From there to about 200 degrees the cell is over temperature but not yet reacting; an onset window sits between roughly 200 and 270 degrees, and above that runaway is self-sustaining, peaking at 400 to 500 degrees. https://bess.courses/glossary/time-synchronization/ https://bess.courses/figures/time-synchronization.png Eight synchronisation accuracies spanning eleven decades on a logarithmic time scale, from a forty-nanosecond GPS broadcast offset through microsecond precision-time-protocol and millisecond relay records, out to a free-running clock drifting 86 milliseconds a day, the 18-second GPS-to-UTC offset, and whole-hour daylight-saving errors. https://bess.courses/glossary/tolling-agreement/ https://bess.courses/figures/tolling-agreement.png A ledger with a boundary down the middle: market and price risk, dispatch decisions and charging energy cross to the toller, while availability, round-trip efficiency, degradation and augmentation cost — and any gap between the toll's cycling rights and the battery warranty — stay with the owner. https://bess.courses/glossary/total-harmonic-distortion/ https://bess.courses/figures/total-harmonic-distortion.png A voltage distortion scale from zero to eight per cent at the point of common coupling, on the IEEE 519 limits for the 1 to 69 kV band where most utility BESS connect. Below three per cent both caps are met. Between three and five, a reading that belongs to a single harmonic order has exceeded its three per cent limit, while a total sitting in the same band has not yet exceeded five. https://bess.courses/glossary/transformer-paralleling/ https://bess.courses/figures/transformer-paralleling-1.png Two loading bars either side of a 100 per cent nameplate line: the 6 per cent transformer reaches about 114 per cent while the 8 per cent transformer sits at about 86 per cent, the gap below the line marked as stranded headroom. https://bess.courses/figures/transformer-paralleling-2.png Circulating current between two 6 per cent transformers at no load: a one per cent ratio gap drives about 8.3 per cent of rated current, and a single 2.5 per cent tap step about 21 per cent. https://bess.courses/glossary/transformer-turns-ratio/ https://bess.courses/figures/transformer-turns-ratio.png One 34.5 kilovolt to 690 volt transformer's 6 per cent impedance marked twice on a logarithmic ohms scale: about 8.3 milliohms seen from the low-voltage side, and about 20.7 ohms seen from the high-voltage side — a factor of 2,500. https://bess.courses/glossary/transformer-vector-group/ https://bess.courses/figures/transformer-vector-group.png Dyn11 read letter by letter: D is the high-voltage delta winding, y the low-voltage star winding, n the low-voltage neutral brought out, and 11 the clock number, which is 11 times 30 degrees. On the clock face the HV phasor points at 12 and the LV phasor at 11, a displacement of 30 degrees. https://bess.courses/glossary/transformer/ https://bess.courses/figures/transformer.png Three voltage levels across one plant with a transformer drawn between each pair: the PCS terminals at 0.4 to 0.8 kV AC, the array collection network at roughly 10 to 35 kV, and the point of interconnection at 60 kV and above. The first machine is the unit step-up transformer, one per PCS block; the second is the station GSU, or main power transformer, at the project substation. https://bess.courses/glossary/transmission-system-operator/ https://bess.courses/figures/transmission-system-operator.png A four-step delegation chain: EU regulation sets frameworks and ceilings, the synchronous area fixes the frequency-containment properties, the load-frequency-control block sets the restoration activation times, and the connecting transmission system operator publishes the requirements the plant actually has to meet. https://bess.courses/glossary/transmission-voltage-and-distance/ https://bess.courses/figures/transmission-voltage-and-distance.png Line loss against distance on logarithmic axes for 33, 132 and 400 kV carrying the same power on the same conductor. The three lines are parallel, separated by the square of the voltage ratio between them: at 200 km the illustrative losses are 11 percent, 0.69 percent and 0.07 percent. The power and the conductor are both arbitrary; the ratios between the lines are the point, not the levels. https://bess.courses/glossary/triplen-harmonics/ https://bess.courses/figures/triplen-harmonics.png Four bars of transformer winding eddy loss as a percentage of the loss the fundamental alone causes, against a dashed reference at that fundamental loss. Winding eddy loss scales roughly with order times amplitude, squared, so a third harmonic at 10 per cent per phase adds 9 per cent, at 20 per cent adds 36, and at 30 per cent adds 81 — nearly doubling the loss. https://bess.courses/glossary/ul-1741/ https://bess.courses/figures/ul-1741.png Four steps: IEEE 1547-2018 states the interconnection requirements, IEEE 1547.1-2020 supplies the test procedures, UL 1741 SB is the product listing demonstrating those functions, and that listing fills the power-conversion slot of a UL 9540 system listing. https://bess.courses/glossary/ul-1973/ https://bess.courses/figures/ul-1973.png Three boxes in a row — cell, module, pack or rack — with one bracket spanning all three labelled UL 1973 Edition 3, and a second, thinner bracket spanning exactly the same three labelled IEC 62619. The point is that one standard now covers the whole span, and a second standard covers the same ground without being interchangeable with it. https://bess.courses/glossary/ul-9540/ https://bess.courses/figures/ul-9540.png Two component standards — UL 1973 for the battery from cell to rack, and UL 1741 for the power conversion system — feeding a single UL 9540 listing of the energy storage system evaluated as one integrated unit. https://bess.courses/glossary/ul-9540a/ https://bess.courses/figures/ul-9540a.png Three documents set side by side and separated by is-not signs rather than arrows: UL 9540A is a test method producing a report with no pass or fail, UL 9540 is a product certification, and NFPA 855 is an installation standard. https://bess.courses/glossary/un-38-3/ https://bess.courses/figures/un-38-3.png An eight-by-five grid of the UN 38.3 transport tests against the article types that take them. Tests T.1 to T.5 apply to every column. T.6 impact or crush is cell-level only, T.7 overcharge is taken by rechargeable batteries only, and T.8 forced discharge applies to lithium cells alone — sodium-ion cells do not take it. https://bess.courses/glossary/unplanned-outage/ https://bess.courses/figures/unplanned-outage.png The same event — one of ten power blocks out for a month — scored by two metrics: a capacity-weighted availability metric loses about 0.83 points, while a time-based metric with a partial-capability threshold reads close to zero. https://bess.courses/glossary/upper-explosive-limit/ https://bess.courses/figures/upper-explosive-limit.png A hydrogen-in-air concentration axis: below about 4 per cent the mixture is too lean to burn, from 4 to 75 per cent it is flammable, and above 75 it is too rich — but only conditionally, since dilution with fresh air walks it straight back down through the flammable range. https://bess.courses/glossary/usable-energy/ https://bess.courses/figures/usable-energy.png A battery pack from 0 to 100 percent state of charge, with the cycled band shaded. Usable energy is the energy contained in that shaded band — depth of discharge times present capacity — which is smaller than nameplate and smaller again at the point of interconnection. https://bess.courses/glossary/vdc-window/ https://bess.courses/figures/vdc-window.png A DC bus voltage scale from 800 to 1,550 volts for a 1500 volt-class LFP system, read at the PCS DC terminals. Two spans that are often conflated are drawn apart. The converter's ACCEPT range starts at about 850 volts, below which it will not take the string at all, and from there up it accepts and derates roughly linearly because the limit is DC current. https://bess.courses/glossary/virtual-power-plant/ https://bess.courses/figures/virtual-power-plant.png A decade-long timeline of FERC Order 2222 implementation. The order was issued in September 2020; NYISO opened an aggregation program in April 2024 and CAISO went live in November 2024. Three further openings are announced but have not happened: ISO-NE in November 2026, PJM in February 2028, and MISO and SPP toward 2029 to 2030, drawn as a window rather than a date. https://bess.courses/glossary/voltage-drop/ https://bess.courses/figures/voltage-drop.png Two bars comparing per-cent voltage drop on the same conductor run at the same power, one per DC voltage class. At 1500 volts the drop is the baseline. At 1000 volts it is 2.25 times larger — the square of fifteen hundred over a thousand — because at fixed power the current scales as one over voltage and the per-cent drop as one over voltage squared. https://bess.courses/glossary/voltage-headroom/ https://bess.courses/figures/voltage-headroom.png Four bars of the full-power DC minimum for one Power Electronics Freemaq family, one per AC output variant: 849 volts at 600 V AC, 934 at 660, 976 at 690 and 1,019 at 720 — each within a volt of the square root of two times the line-to-line AC voltage. A dashed reference line sits at 900 volts, the cold and loaded corner a 1500 volt-class string's protected floor can reach. https://bess.courses/glossary/voltage/ https://bess.courses/figures/voltage.png Two ladders, not one. On the DC side a 3.2 V LFP cell stacks into a 104-cell module near 330 V and a string of four such modules — 416 cells in series — working across 1,100 to 1,500 V. The PCS converts, and the AC ladder starts over from its own base: the inverter output at 400 to 800 V, medium-voltage collection around 34.5 kV, and high voltage at the point of interconnection. https://bess.courses/glossary/weakest-cell-limitation/ https://bess.courses/figures/weakest-cell-limitation.png Three bars of usable energy through one 416-cell series rack: the best cell a little above average, the average of all 416 — which is what a state-of-health report shows — and the worst cell, drawn as the reference because it is the rack's actual operating window. https://bess.courses/glossary/weighted-average-cost-of-capital/ https://bess.courses/figures/weighted-average-cost-of-capital.png An annual cost-of-capital axis showing the after-tax cost of debt at roughly 4 to 7 per cent, the cost of equity at roughly 9 to 14, and the blended weighted average between them at about 6 to 9 — closer to the debt end, because debt is the larger share of the stack. https://bess.courses/glossary/wye-star-connection/ https://bess.courses/figures/wye-star-connection.png Three windings joined at a common neutral point, their far ends brought out as line terminals A, B and C. The neutral node is drawn bare, with nothing attached below it — the connection creates the point but earths nothing. https://bess.courses/glossary/zero-sequence/ https://bess.courses/figures/zero-sequence.png Three sets of three phasors drawn as arrows from a common origin. The third is emphasised: the zero-sequence set, three equal arrows with no displacement between them at all — fanned very slightly so that three are still visible as three — and no rotation, against the positive-sequence set turning a-b-c and the negative-sequence set turning a-c-b, both 120 degrees apart. https://bess.courses/project-development/ https://bess.courses/figures/project-stage-machine.png Nine rails read top to bottom: five development stages of a grid-scale battery project — site, permitting, interconnection, off-take and financing — then two gate rails, sponsor and lenders, then construction and commissioning.