Amp-hour capacity Ah
Amp-hour capacity is the charge a battery cell can deliver — current integrated over time, measured in ampere-hours (Ah). It is not energy: a 314 Ah LFP cell holds roughly 1.0 kWh only after multiplying by its ~3.2 V nominal voltage, and that multiplication is an approximation, because cell voltage moves with state of charge, current and temperature.
Cell datasheets rate capacity in Ah and the industry names its cell formats by it — 280 Ah, 314 Ah, 500+ Ah — while system datasheets quote MWh, because series connection raises voltage without touching amp-hours, so only energy aggregates cleanly from cell to container. C-rate, the unit every charge and discharge rate is stated in, is defined against this number.
Reviewed August 2026 by Sergey Syrvachev
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What it is (precise)
Amp-hour capacity counts charge. One ampere flowing for one hour moves 3,600 coulombs, so a 314 Ah rating says the cell can pass about 1.13 million coulombs between full and empty. Electrochemistry keeps its books in exactly this currency: every lithium ion shuttled between the electrodes carries one elementary charge, matched by one electron through the external circuit, so amp-hours are a direct measure of the working lithium inventory the cell can cycle.
That is why the headline number on a cell datasheet is Ah rather than Wh — it describes how much active material the cell brings to work, separate from the voltage the chemistry happens to develop across it.
The rating is a conditioned measurement, not a constant of nature. Rated capacity is the amp-hours delivered by a constant-current discharge between the cell's voltage limits — roughly 3.65 V down to 2.5 V for stationary LFP — at a stated temperature and reference rate, commonly 25 °C.
Discharge colder or harder and the terminal voltage sags into the cutoff earlier, so the measured amp-hours shrink even though the lithium inventory has not changed; the charge left behind is recoverable at a gentler rate. The industry names its cell formats by the rated figure: 280 Ah was the stationary workhorse for years, roughly 314 Ah is the current mainstream, and 500+ Ah formats are entering production — about 0.9-1.0 kWh per cell for the mainstream sizes at ~3.2 V nominal.
Ah × voltage ≈ Wh — and where the approximation breaks
Multiply amp-hours by nominal voltage and you get energy, approximately: 314 Ah × 3.2 V ≈ 1.0 kWh. The approximation is baked into the datasheet itself — the energy row a cell datasheet prints is derived from the Ah row this way. But nominal voltage is a single representative marker on a curve the cell traverses from ~3.65 V full to 2.5 V empty, and the energy a discharge actually delivers is the integral of voltage times current over that whole trajectory.
The trajectory shifts with operating point: at higher current the resistive sag pulls the entire curve down, and in the cold it sags further, so the same amp-hours yield fewer watt-hours discharged hard or cold. That is one reason a capacity test states both quantities, and why Nominal energy — amp-hours times nominal voltage times cell count — is an arithmetic construct no meter ever reads.
The approximation also hides where round-trip loss lives. A lithium-ion cell returns charge almost perfectly — amp-hours out very nearly equal amp-hours in — but charging rides a higher voltage curve than discharging, because resistive sag adds to terminal voltage on the way in and subtracts on the way out.
So the watt-hours refuse to balance even when the amp-hours do: the area between the charge and discharge curves is heat, and it grows with current. Efficiency is always an energy statement, never a charge one, which is why Round-Trip Efficiency is defined in Wh at a stated boundary rather than in Ah.
An ampere-hour is 3,600 coulombs: current integrated over time, with no voltage in it anywhere. Cells are named by it — 280 Ah the long-time stationary workhorse, ~314 Ah the current mainstream, 500+ Ah entering production — and rated by constant-current discharge between voltage limits, roughly 3.65 down to 2.5 V for LFP, at a stated temperature and reference rate. The aggregation rule is the tidy part: series adds voltage at constant Ah, parallel adds Ah at constant voltage, and only energy accumulates through both. C-rate anchors to it too: a 314 Ah cell at 0.5C carries about 157 A. Where the Wh ≈ Ah × V approximation breaks is under load — delivered energy is the integral of a voltage curve that sags with current and cold, so the same amp-hours yield fewer watt-hours discharged hard or cold.
- Definition
- Charge, not energy — current integrated over time, in ampere-hours; 1 Ah = 3,600 coulombs
- The conversion
- Wh ≈ Ah × nominal voltage: 314 Ah × ~3.2 V ≈ 1.0 kWh for LFP; NMC stores more energy per Ah at ~3.6-3.7 V nominal
- Mainstream formats
- Cells are named by their Ah: 280 Ah the long-time stationary workhorse, ~314 Ah current mainstream, 500+ Ah entering production
- Rating conditions
- Rated Ah = constant-current discharge between voltage limits (~3.65 to 2.5 V for LFP) at a stated temperature and reference rate
- Aggregation
- Series adds voltage at constant Ah; parallel adds Ah at constant voltage; only energy accumulates through both
- C-rate anchor
- Cell-level C-rate is current over rated Ah: a 314 Ah cell at 0.5C carries ~157 A
- Where the approximation breaks
- Delivered Wh is the integral of a voltage curve that sags with current and cold — the same Ah yields fewer Wh discharged hard or cold
- Not the same as
- Energy (Wh at a stated boundary), Nominal energy (Ah × V × cell count, a paper construct), C-rate (a rate defined against Ah)
Why cells count charge and systems count energy
The unit changes as the hierarchy climbs because the two quantities aggregate differently. Series connection stacks voltage while amp-hours stay fixed — every cell in a string passes the same current, so the string's Ah rating is the cell's, and the weakest cell sets it. Parallel connection adds amp-hours at a common voltage.
Only their product, energy, accumulates through both — the standard 416S, 314 Ah rack comes out at about 418 kWh — series connection walks the arithmetic — and a container of such racks in parallel is quoted in MWh. An amp-hour figure is meaningless as a system rating without the voltage architecture attached; a megawatt-hour figure needs no such qualifier, which is why procurement, warranties and markets all speak MWh from rack level up.
The split runs through operations too. The BMS estimates State of Charge by coulomb counting — integrating current to track amp-hours in and out — because charge is the quantity the cell conserves between calibrations. The revenue meter at the point of interconnection integrates power into MWh, because energy is what markets settle.
C-rate bridges the two frames: at cell level it is current divided by rated amp-hour capacity, so a 314 Ah cell at 0.5C carries roughly 157 A, while system engineering restates the same idea as power over energy. The two definitions drift apart as voltage sags under load — 1C in amps is not exactly 1C in watts — and what that divergence costs in heat, duration and voltage stress is the C-rate page's story.
Common pitfalls
The classic error is multiplying by the wrong voltage. Amp-hours convert to energy through the nominal voltage of the exact assembly being quoted: cell Ah times cell nominal for a cell, times string nominal for a string.
A 1P104S module of 314 Ah cells is still a 314 Ah module — its roughly 100-105 kWh comes from the ~330 V module voltage, not from any summing of amp-hours — and adding up the Ah of series cells double-counts what the series connection already expressed as voltage. The reverse confusion appears in sizing spreadsheets that divide a container's MWh by cell Ah alone and land on a cell count that ignores the series-parallel architecture entirely.
Amp-hours also fail as a comparison unit across chemistries and conditions. An NMC cell of identical Ah stores more energy than its LFP counterpart in proportion to its higher nominal voltage — ~3.6-3.7 V against ~3.2 V — so a cost-per-Ah comparison silently penalizes the higher-voltage chemistry and a capacity-per-Ah one flatters it.
Rated figures only compare at matching conditions: the same cell measures more amp-hours at a gentler rate or warmer temperature, so check the reference C-rate and temperature before ranking datasheets. Degradation reporting inherits the whole distinction — cell test reports track Ah fade, system capacity tests measure MWh at a meter, and a Capacity retention clause must name which quantity, at which boundary, it is written against.
Amp-hours and watt-hours are the same information in different units — multiply or divide by voltage and you convert freely, so a 314 Ah cell simply is a 1 kWh cell.
In reality: The conversion holds only at a single fictitious operating point. Nominal voltage is a marker on a curve the cell traverses from ~3.65 V full to 2.5 V empty, and the watt-hours a discharge actually delivers are the integral of voltage times current along that trajectory — lower at high rate and low temperature because the whole curve sags, even while the amp-hours delivered barely change. The gap cuts the other way between chemistries: an NMC cell of identical amp-hours stores more energy than an LFP cell purely because its nominal voltage is higher. Ah tells you how much charge the cell can shuttle; only a stated voltage trajectory turns that into energy, which is why capacity tests report both and why the money is settled in MWh.
- Energy Glossary
- C-rate Glossary
- Sodium-Ion vs LFP for Grid Storage — and Where NMC Still Fits Article
- Interactive: Lithium-Ion Cell Interactive visual · bess.engineer
Amp-hour capacity, in context.
The Grid-Scale BESS course covers amp-hour capacity — and the rest of the system — from the ground up, the way it actually gets deployed.