Usable vs nameplate kWh: why two 100 kWh batteries are not the same size
A 100 kWh battery rarely delivers 100 kWh. Usable energy is nameplate capacity multiplied by the depth of discharge the supplier guarantees, then reduced by temperature, ageing and the voltage window the BMS enforces. Two identically labelled cabinets can differ by 10–20% in dispatchable energy on day one, and by more at year ten. Ask for usable kWh at beginning of life and at end of warranty, in writing, before comparing any $/kWh figure.
The short answer
Nameplate kWh is the nominal energy stored in the cells when new, measured under the test conditions on the datasheet. Usable kWh is the energy you can actually dispatch through the AC terminals within the guaranteed operating window. The bridge between them is depth of discharge (DoD):
Usable kWh = Nameplate kWh x DoD (%) / 100
| Nameplate | Guaranteed DoD | Usable energy (new) | Difference vs 95% case |
|---|---|---|---|
| 100 kWh | 80% | 80 kWh | -15.8% |
| 100 kWh | 90% | 90 kWh | -5.3% |
| 100 kWh | 95% | 95 kWh | baseline |
| 112 kWh | 90% | 100.8 kWh | +6.1% |
The fourth row is the one that causes procurement errors. A supplier quoting a larger nameplate at a lower DoD can deliver more energy than a smaller cabinet at a higher DoD, but only if the comparison is made on usable figures. If you divide both prices by the number printed in the product title, the ranking will be wrong.
Where the difference comes from
1. The BMS voltage window
The battery management system prevents cells from reaching their absolute maximum and minimum voltages. Operating across the full electrochemical range would shorten cycle life sharply, so manufacturers define a narrower permitted state-of-charge window. That window is the DoD figure in the warranty, not a suggestion.
2. Temperature and power derating
Datasheet capacity is normally quoted at around 25°C and a defined C-rate. Outdoor cabinets in a hot climate, or high-power discharge in a cold one, deliver less energy. Ask whether the quoted usable figure holds across the site’s ambient range, and whether HVAC parasitic load is included in the round-trip efficiency figure.
3. The cycle-life trade-off
A deeper discharge extracts more energy per cycle and typically consumes more of the cell’s life. This is why a supplier offering 6,000 cycles might quote 90% DoD while another quotes 80% for a longer calendar life. Neither is inherently better; they are different products. The problem starts when the two are compared on price per nameplate kWh.
4. Ageing and end-of-warranty capacity
Capacity falls with cycling and calendar time. Many C&I warranties express a capacity-retention floor at the end of the term — commonly around 80% of initial capacity over a 10-year term — or a total energy-throughput allowance, whichever comes first. A system that starts with 90 kWh usable may be warranted for noticeably less by year ten. Model the degraded figure, not the day-one figure.
5. Where the measurement is taken
Energy can be specified at the cell, at the DC terminals, or at the AC terminals. Conversion losses in the PCS sit between the last two. Two suppliers can both claim 90% round-trip efficiency and mean different things. Require the measurement boundary in writing.
A worked example with three quotations
Assume three suppliers responded to the same request for a roughly 100 kWh system. These are illustrative figures, not market prices.
| Supplier | Price (USD) | Nameplate | DoD | Usable | USD / usable kWh | Scope note |
|---|---|---|---|---|---|---|
| A | 42,000 | 100 kWh | 90% | 90.0 kWh | 467 | PCS included; BOS excluded |
| B | 51,000 | 100 kWh | 95% | 95.0 kWh | 537 | PCS and commissioning included |
| C | 46,000 | 112 kWh | 90% | 100.8 kWh | 456 | PCS excluded; freight included |
On nameplate figures the ranking is A (420 $/kWh), B (510), C (411). On usable figures it is C (456), A (467), B (537). Both rankings are still wrong, because C excludes the PCS and A excludes installation and commissioning. The usable-capacity correction is necessary but not sufficient: scope must be normalised in the same pass. The quote comparison sheet does both on one screen and lets you export the result.
What to write into the RFQ
Do not ask for “100 kWh”. Ask every supplier to complete the same columns, with a guaranteed value, the test method and the supporting document for each. That way the returned quotations can be compared line by line rather than reinterpreted.
- Usable kWh at beginning of life, and at the end of the warranty term
- Depth of discharge assumed, and the permitted state-of-charge window
- Round-trip efficiency with its measurement boundary (cell, DC or AC)
- Assumed annual degradation and the year-10 capacity
- Warranty term, throughput or cycle cap, and capacity-retention floor
- Ambient temperature range over which the usable figure holds
- Whether HVAC and auxiliary consumption are included in the efficiency figure
- Whether augmentation is expected, and who pays for it
The ESS supplier RFQ checklist sets out the wider 12-item specification framework. Add a mandatory exceptions section so that any requirement a supplier cannot meet is declared with its commercial impact rather than buried in the small print.
How this changes the economics
Financial models for peak shaving and time-of-use shifting need installed cost and dispatchable energy as inputs. If the cost input excludes the PCS while the energy input is nameplate rather than usable, the payback calculation will be optimistic in two directions at once. Fix the definition of the kWh first, fix the scope second, and only then run the ROI model. A generic ROI calculator cannot do this for you, because it has no way of knowing which scope items a given supplier left out.
For system context, see the 50–100 kWh industrial battery storage guide, and for the full set of buyer material see the resources hub.
Frequently asked questions
Is a 100 kWh battery always 100 kWh?
No. 100 kWh is the nominal cell capacity under datasheet conditions. The energy available for dispatch is lower because the BMS restricts the state-of-charge window to protect cycle life, and lower again after conversion losses, temperature derating and ageing. Always ask for guaranteed usable kWh at both beginning and end of the warranty term.
What depth of discharge should I specify?
There is no universal correct number. LFP C&I systems commonly quote 90–95% DoD, but a lower figure may accompany a longer cycle-life or calendar-life guarantee. Specify the usable energy you need and ask each supplier to state the DoD, cycle count and capacity-retention floor that delivers it. Compare the whole package, not the percentage in isolation.
Should I compare $/kWh or $/usable kWh?
Use $/usable kWh as the first normalisation step, because it removes capacity-definition differences. It is still not a purchase decision: a quote that excludes the PCS, installation, freight or duties will look cheap on any per-kWh basis. Add the missing scope back in before ranking suppliers.
How does a throughput warranty change the comparison?
A warranty may expire on years or on total energy throughput, whichever comes first. Two systems with identical usable capacity and price can have very different lifetime cost if one allows twice the throughput. Ask for the throughput cap, the capacity-retention floor at that point, and the cost of augmentation once the floor is reached.
General information only, current as of November 2026, and not engineering, financial, legal or fire-protection advice. Figures used in the worked example are illustrative. Applicable standards and grid codes are adopted and amended locally; the authority having jurisdiction and your grid operator determine what applies at your site. Confirm all values with the supplier in writing before procurement.
