About the Battery Backup Calculator
This battery backup calculator sizes a battery for the appliances you want to keep running during an outage, a camping trip or an off-grid night. Enter the combined power draw of your essential loads and how many hours they must run, and it returns the battery capacity you need in kilowatt-hours and amp-hours at your system voltage, allowing for inverter losses and a safe depth of discharge.
It also works the other way: enter the capacity of a battery you already own or are considering — a home battery, portable power station or UPS — to see how long it would run the same load. The chart shows how runtime changes if you run fewer or more devices.
Lithium iron phosphate (LFP) batteries are typically used down to 80–100% depth of discharge, while lead-acid batteries should stay around 50% for a long life. Inverters usually convert 85–95% of battery energy into AC power. Motors and compressors (fridges, well pumps) can briefly draw two to three times their running watts at start-up, so check the inverter’s surge rating too.
With the default inputs, the battery capacity needed is 7.9 kWh. Change any value above to recalculate instantly.
How to use the battery backup calculator
- 1List the devices you need during an outage and add up their running watts.
- 2Enter how many hours of backup you want.
- 3Pick your battery voltage, depth of discharge and inverter efficiency.
- 4Read the capacity needed in kWh and Ah.
- 5Optionally enter an existing battery’s kWh to see how long it would last.
Formula and method
The energy your devices consume is load × hours. Because the inverter wastes some energy converting DC battery power into AC, the battery must supply that energy divided by the inverter efficiency. And because you should not drain a battery completely, the rated capacity is the energy needed divided by the usable depth of discharge.
Amp-hours are watt-hours divided by the battery system voltage, which is how many battery banks are labelled. Runtime for an existing battery reverses the formula: rated watt-hours × depth of discharge × inverter efficiency ÷ load. Real runtime is also affected by temperature, battery age and discharge rate (especially for lead-acid), so leave some margin.
- Load
- Combined running power of the devices in watts
- DoD
- Usable depth of discharge (0.9 = 90%)
- eff.
- Inverter efficiency (0.9 = 90%)
- V
- Battery bank voltage (12, 24 or 48 V)
Worked examples
Essentials for 8 hours on a 48 V LFP bank
An 800 W load for 8 hours uses 6,400 Wh; after 90% inverter efficiency the battery must deliver 7,111 Wh, and at 90% depth of discharge that means 7.9 kWh of capacity, or about 165 Ah at 48 V. A 13.5 kWh battery would run the same load for about 13.7 hours.
CPAP and lights overnight on a 12 V lead-acid battery
Sixty watts for ten hours is 600 Wh, or 706 Wh from the battery at 85% efficiency. Using only half of a lead-acid battery, you need 1.41 kWh — about 118 Ah at 12 V. A 1.2 kWh (100 Ah) battery would last only 8.5 hours.
Whole-home backup for 24 hours
A 1.5 kW average load for a day needs 36 kWh, or 37.9 kWh of battery at 95% efficiency and full depth of discharge. Two 13.5 kWh batteries (27 kWh) would last about 17 hours.
Frequently asked questions
How big a battery do I need to power my house?+
Add up the watts of the essentials you want to run, multiply by the hours of backup, then divide by inverter efficiency (about 0.9) and depth of discharge. Many homes back up essentials with 10–15 kWh; running central AC or electric heat needs far more.
How long will a 10 kWh battery last?+
Divide the usable energy by your load. A 10 kWh battery with 90% usable capacity and a 90% efficient inverter delivers about 8.1 kWh, so it runs a 500 W load for roughly 16 hours or a 1,000 W load for about 8 hours.
How do I convert amp-hours to watt-hours?+
Multiply amp-hours by the battery voltage. A 100 Ah battery at 12 V stores 1,200 Wh (1.2 kWh); the same 100 Ah at 48 V stores 4,800 Wh. Compare batteries in watt-hours, not amp-hours, when voltages differ.
What depth of discharge should I use?+
Lithium iron phosphate batteries tolerate 80–100% depth of discharge for thousands of cycles. Lead-acid and AGM batteries last much longer if kept to about 50%. Manufacturers’ usable capacity figures often already include this limit.
Why do I need a bigger inverter than my load?+
Inverters should not run continuously at 100% of their rating, and motors in fridges, pumps and compressors draw a surge of two to three times their running power when starting. A 25% margin plus an adequate surge rating avoids trips.