The formula
A worked example
500 W for 12 hours needs 6 kWh delivered. With an 80% operating window and 90% efficiency, the arithmetic minimum is about 8.33 kWh of usable capacity.
What this estimate includes
The operating window is the portion of usable capacity you plan to draw, after reserve restrictions. This is an energy calculation rather than equipment sizing approval; inverter output and installation requirements must also be checked.
Common questions
Is the result usable or nameplate capacity?
The result is required usable capacity before applying your operating window. Manufacturer nameplate capacity may be different.
Should I add a margin?
Real loads vary and capacity changes over time. Compare measured demand and manufacturer specifications before choosing equipment.
Further reading
Energy Saving Trust: home battery storage. Background reference; these tools are independent and are not endorsed by the source organization.