Home battery size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Electricity you use a day | 8 kWh |
| Share used after sunset | 60 % |
| Depth of discharge | 90 % |
Battery: about 5.33 kWh nominal (4.80 kWh usable)
| Result | Value |
|---|---|
| Nominal battery capacity (indicative) | 5.33 kWh |
| Usable capacity needed | 4.8 kWh |
Show the working
- Usable capacity: 8 kWh × 60% = 4.8 kWh
- Nominal capacity: 4.8 kWh ÷ 90% = 5.33 kWh
How it works
- Usable capacity = daily use (kWh) × share used after sunset
- Nominal capacity = usable capacity ÷ depth of discharge
- Energy to charge it = usable capacity ÷ round-trip efficiency
Batteries come in set sizes: pick the next size up from the maker's range.
BS 7671:2018+A4:2026 adds requirements for battery energy storage; the installer follows them and the maker's instructions.
Indicative only: an installer checks your usage data and the supply.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- BS 7671:2018+A4:2026, new requirements for battery energy storage systems (Researcher to cite the chapter)
Explained: BS 7671 (the IET Wiring Regulations), Approved Document P (electrical safety).
BS 7671 is used across the UK.