The BESS sizing calculator. The waterfall, interactive.

Enter a contract — power, duration, guarantee year — and watch it walk backward through every real-world loss to the DC nameplate you'd actually have to buy. The same method as the sizing guide, with your numbers — and the same math behind our computed analysis of the BOL overbuild.

Assumptions

Contract
Loss chain · POI → cell
Excludes only the BMS end reserve; with BOL fade and cell efficiency, 95.5% of nameplate leaves the cells.
Warranty & power side

632.7 MWh of DC nameplate, 1.58× the POI requirement

The DC nameplate to install for 400 MWh deliverable at the POI (100 MW × 4 h), sized so it still delivers in year 20.
442.9 MWh if you only sized for day one (1.11× — the loss chain alone, no degradation)
0.16 C discharge C-rate against the installed pack
105.3 MVA → 30 × 3.6 MVA PCS fleet: converters are sized in MVA, not MW — at PF 0.95, 100 MW needs 105.3 MVA (108 MVA installed)

The energy waterfall

Every step as a table, with the reasoning
The energy waterfall, step by step
StepAdds (MWh)Running total (MWh)
Contract: deliverable at the POIDeliverable energy the offtaker is promised at the point of interconnection.400
HV transformer (POI is on the HV side)The offtake meter sits on the HV side, so the step-up transformer is the first loss you buy back — 99.5% on the discharge leg.+2.0402
AC cablingMV/HV AC runs and terminations between the transformers.+1.2403.2
Auxiliary / ancillary loadsThermal management, controls, and standby draw ~2% of throughput — a serious model books this as its own line, not a footnote.+8.2411.4
MV transformerThe medium-voltage step-up between the PCS and the collection bus.+4.2415.6
PCS conversionThe inverter's own conversion loss on the discharge leg.+6.3421.9
DC cablingCable runs and fusing from the racks to the PCS.+1.3423.2
Usable SoC window (DoD)The BMS reserves headroom at the ends of the SoC range; only 99% of installed energy is dispatchable.+4.3427.5
Production-to-BOL cell fadeCells lose a little capacity between the factory and first commissioning.+2.1429.6
Cell one-way efficiencyThe cell's own round-trip loss, counted on the discharge half.+13.3442.9
Day-one totalThe loss chain alone, no degradation.442.9
Degradation to year 20 (SOH 70%)Size so the fleet still clears the contract in year 20, once it has faded to 70% of BOL — unless you augment mid-life instead of overbuilding on day one.+189.8632.7
DC nameplate you'd have to buySized so the plant still delivers the contract in the guarantee year.632.7
Guarantee-point sentence: 400 MWh deliverable at the POI, at 100 MW, in year 20, after the full HV-transformer-to-cell loss chain, a 99% SoC window, and 70% SOH.
First-pass educational estimate. Real projects add temperature/altitude derating, cycling-dependent degradation curves, augmentation strategies, and vendor-specific capability data — exactly what the sizing guide, the overbuild analysis, and the full course teach.

Why the nameplate is bigger than the contract

The contract is measured at the POI, the meter on the high-voltage side of the plant. Every loss between the cells and that meter is energy the battery has to supply on top, so it is bought back as extra DC nameplate.

The PCS converts the battery's DC to grid AC. Its fleet is counted in MVA, not MW, because it must also hold the power factor the grid code asks for.

The BMS keeps the cells inside a usable SoC window, so a slice of the installed energy is never dispatched.

Cells start a little below their factory rating at BOL and keep fading. The guarantee is written against the SOH in the guarantee year, which is why degradation is usually the longest bar.

Link or cite this tool

Free to use and link. Citation: BESS Sizing Calculator, BESS.courses — https://bess.courses/tools/bess-sizing/