About the Battery Backup Calculator
Battery capacity needed to back up your home for a set number of hours.
Also known as: home battery size, backup power calculator, power cut battery, whole house battery.
How to use this calculator
- Enter average load to back up, backup hours needed, battery bank voltage, usable depth of discharge and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
kWh = load × hours ÷ efficiency ÷ depth of discharge
Worked example
Using the values pre-filled in the calculator above:
- Average load to back up = 800 W
- Backup hours needed = 8
- Battery bank voltage = 48 V
- Usable depth of discharge = 80 %
- Inverter efficiency = 92 %
- Battery cost per kWh = 400
the calculator returns:
- Battery capacity needed: 8.696 kWh
- In amp-hours: 181.2 Ah
- Energy delivered to the load: 6.957 kWh
- Discharge current: 18.12 A