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Usable capacity, nominal capacity and efficiency of a battery
Updated 13 Oct 2026 · KilowattTally
Two “8 kWh” batteries can deliver different amounts of energy to the house. Before entering a capacity in the calculator, it helps to know which data sheet figure you are reading.
Nominal and usable
- Nominal capacity: the energy stored in the cells. Not all of it can be used: reserves for ageing and for safety, and protection against deep discharge, reduce it.
- Usable capacity (in German Nutzkapazität): the energy that can be drawn during discharge. This is the figure to compare and to enter in the calculator.
- Depth of discharge: the ratio between usable and nominal. A battery used between 5% and 95% state of charge has a depth of discharge of 90%.
HTW Berlin, in the Stromspeicher-Inspektion 2026, gives two examples [6]. One data sheet states 16 kWh and a depth of discharge of 90%: the usable capacity is 14.4 kWh. For a BYD HVS+ 7.7, the battery manufacturer declares 7.7 kWh usable with 100% depth of discharge, but the inverter manufacturer limits it to 95% and declares 7.3 kWh; in the laboratory HTW measured 7.4.
How far data sheets differ from measurements
Of 12 systems tested, 11 state the usable capacity on the battery data sheet. The usable capacity measured in the laboratory differed from the data sheet by −7 to +8 percentage points, and for 7 of the 12 systems the measurement was higher than the data sheet [6]. The measured usable capacities ranged from 6.4 to 7.6 kWh for four smaller systems and were above 11.9 kWh for five systems [6].
Efficiency
The energy that comes back from the battery is less than the energy that goes in. HTW measures the efficiency for each path [6]:
| Path | Measured values |
|---|---|
| Average efficiency of the batteries alone | from 87.9% to 97.1%; 6 systems out of 12 above 95.7% |
| Charging from photovoltaics (hybrid systems) | up to 98.2%; the lowest 92% |
| Discharging to the house | from 91.2% to 97.8% |
The calculator uses 90% for the whole round trip, charging and discharging together.
How much they affect self-sufficiency
We repeated the calculation for a house using 4,500 kWh in a year with 6 kWp facing south, with the BDEW load profile [1] and the PVGIS weather year [4], changing only the round-trip efficiency, with a battery of 6 kWh usable:
| Round-trip efficiency | Self-sufficiency | From the grid in a year |
|---|---|---|
| 80% | 65.5% | 1,553 kWh |
| 85% | 66.2% | 1,521 kWh |
| 90% | 66.9% | 1,491 kWh |
| 95% | 67.5% | 1,464 kWh |
For this house, 15 points of efficiency are worth 2 points of self-sufficiency, while in the same simulation going from 6 to 8 kWh usable is worth about 3, and going from 0 to 6 kWh about 30 (open the calculation). When comparing quotes, then, the first thing is to compare the usable capacity; efficiency changes less, but it still counts towards the kWh over a year. Standby consumption, which HTW measures for each system [6], is not counted here.
Sources
- Repräsentative Profile BDEW H25, G25, L25, P25, S25 (Veröffentlichung) — BDEW Bundesverband der Energie- und Wasserwirtschaft, retrieved 13 Oct 2026
- PVGIS 5.3, serie orarie (seriescalc), banca dati PVGIS-SARAH3 — Commissione europea, Joint Research Centre (JRC), retrieved 13 Oct 2026
- Stromspeicher-Inspektion 2026 — HTW Berlin e aquu, retrieved 13 Oct 2026