Solar Battery Usable Capacity Explained
A battery's usable capacity is the energy you can actually draw each cycle — always less than its nominal (rated) capacity. Two things reduce it: the manufacturer keeps some capacity in reserve (depth of discharge), and some energy is lost charging and discharging (round-trip efficiency). Both matter because payback is driven by usable, not nominal, kWh.
The three numbers that matter
- Nominal (rated) capacity
- The headline kWh figure on the spec sheet. It is not all available to use.
- Usable capacity
- The kWh you can actually cycle, after the reserved portion. This is the figure to enter in a payback calculation.
- Round-trip efficiency
- The share of energy that survives a full charge-then-discharge cycle (some is lost as heat). A common range is in the high-80s to mid-90s percent; use your unit's figure.
Why it changes payback
If you charge a battery from spare solar and discharge it at night, the energy you avoid buying is limited by usable capacity and reduced by round-trip losses. A “10 kWh” battery does not automatically shift 10 kWh of useful energy each day. Our payback calculator asks for usable capacity and efficiency precisely so the estimate is honest.
Capacity also fades over time
Batteries lose a little usable capacity each year (degradation). The payback model lets you enter an annual degradation percentage so later years are not overstated.
What to ask your installer