The formula
Delivered energy = usable capacity × (start − reserve) ÷ 100 × efficiency. Runtime = delivered energy ÷ (load watts ÷ 1,000).
A worked example
10 kWh at 100% charge, a 20% reserve and 90% delivery efficiency provides 7.2 kWh to loads. At a constant 500 W, that is 14.4 hours.
What this estimate includes
This estimates energy duration, not whether an inverter can start or power each appliance. Startup surges, continuous output ratings, aging, temperature and standby consumption can limit actual backup.
Common questions
Will a 10 kWh battery power my whole house?
Capacity alone cannot answer that. You also need load demand, inverter power limits, surge capability and a compatible backup installation.
Does this account for solar charging during an outage?
No. It assumes no new energy enters the battery during the modeled outage.
Further reading
Energy Saving Trust: home battery storage. Background reference; these tools are independent and are not endorsed by the source organization.