Battery Backup Calculator
A Backup Battery Has to Deliver More Than the Load Alone Suggests
Multiplying a load by the hours you want it powered gives the raw energy needed, but it badly undersizes the battery you'd actually need to buy. Batteries can't be drained to zero without damaging them, and every stage of charging and discharging loses some energy to heat and conversion inefficiency. This calculator builds both of those real-world factors into the sizing.
The Formula
Battery Capacity (Wh) = Energy Needed / (Depth of Discharge × System Efficiency)
Battery Capacity (Ah) = Battery Capacity (Wh) / Voltage
Depth of discharge limits how much of the battery's rated capacity can be safely used — many battery chemistries shouldn't be run all the way to empty. System efficiency accounts for inverter and conversion losses between the battery and the actual load.
Where This Is Useful
- Home backup planning — size a battery bank correctly for a target set of appliances and a target outage duration.
- RV and off-grid setups — work out exactly how many amp-hours of battery a given 12V or 24V system needs to run its loads.
- Avoiding premature battery failure — sizing with an appropriate depth of discharge extends battery lifespan compared to routinely draining it too deeply.
Worked Example
A 1,500 W load needed for 8 hours, on a 24V battery bank with 80% depth of discharge and 90% system efficiency:
| Load | Backup hours | Voltage | Depth of discharge | System efficiency | Required capacity |
|---|---|---|---|---|---|
| 1,500 W | 8 h | 24 V | 80% | 90% | 16,666.7 Wh (694.4 Ah) |
How to Use This Calculator
- Enter the Load in watts — the total power draw to be backed up.
- Enter the Backup Hours Needed.
- Optionally enter Battery Voltage — defaults to 12V if left blank.
- Optionally enter Depth of Discharge % — defaults to 80% if left blank.
- Optionally enter System Efficiency % — defaults to 90% if left blank.
- Select Calculate to get the required battery capacity in watt-hours and amp-hours.
Related Calculations
Size the battery bank for a full off-grid setup with the Off-Grid System Size Calculator, or check the cost of that capacity with the Energy Storage Cost Calculator.