Home battery backup calculator
See how long a home battery will keep your essentials running in a power cut. Enter your battery size and the loads you want to back up to get an estimated runtime.
How it works
- Adds up the power draw of the appliances you choose to back up.
- Divides usable battery capacity by that load.
- Shows estimated backup hours for essentials-only or whole-home use.
What changes your result
- Usable battery capacity in kWh
- Which loads you back up — essentials only versus the whole home
- High-draw appliances like AC, electric heat or a range, which drain it fast
- Whether solar can recharge the battery during the day
Frequently asked questions
How long will a home battery last in an outage?
It depends on battery size and what you run. Backing up just essentials — fridge, lights, internet — a typical 10–13 kWh battery lasts 12–24+ hours. Adding heating, AC or an electric range cuts that to a few hours.
How many batteries do I need for backup?
For essentials, one 10–13 kWh battery is often enough overnight. For whole-home or multi-day backup, especially with AC or electric heat, you may need two or more, or pairing with solar to recharge by day.
Can solar recharge a battery during an outage?
Yes, if designed for it. Solar plus battery can island from the grid and recharge by day, extending backup in good weather. Not all grid-tied solar does this without a compatible battery and inverter.