How Big a Home Backup Battery Do I Need?
Size a home backup battery using watts, daily watt-hours, outage duration, surge loads and recharge options.
The right battery size comes from a critical-load energy budget. Start with the devices you refuse to lose during an outage and ignore the rest until the baseline is covered.
What matters
- List each critical device and its running watts.
- Estimate realistic hours of use per day.
- Add daily watt-hours, then multiply by desired outage days.
- Apply an efficiency/reserve factor and separately verify inverter surge capability.
Most sizing errors come from mixing up watts and watt-hours. Use watts for simultaneous load and watt-hours for duration.
Use the numbers before choosing a model
Start with your load list. The battery needs enough watt-hours for the desired runtime and enough continuous/surge watts to operate the appliances at the same time. Then check whether your recharge method can replace the energy you expect to consume.
For outage planning, we recommend keeping reserve capacity instead of designing a system that reaches 0% at the end of the forecasted outage.
Next step
Estimate battery runtime → or Build a critical-load energy budget →.