Battery Backup Time Calculator
Calculate how long a battery bank can support your appliances using voltage, amp-hours, usable depth of discharge, conversion efficiency and inverter losses.
Works with lead-acid, AGM and LiFePO₄ systems. No signup required.
Describe your backup system
Bank Ah means total capacity at the selected voltage. In series, voltage adds but Ah stays the same. In parallel, Ah adds while voltage stays the same.
- Rated bank energy
- 1,200 Wh
- Usable AC energy
- 510 Wh
- Total draw
- 105 W
- Load vs inverter
- 10%
Estimate only. Battery age, temperature, discharge rate, wiring loss and inverter behavior can change actual runtime.
How to calculate battery backup time
Multiply battery-bank voltage by total bank amp-hours, apply the usable-capacity percentage and conversion efficiency, then divide by appliance load plus inverter idle consumption.
1. Rated energy
Voltage × total bank Ah = watt-hours
2. Usable AC energy
Watt-hours × usable capacity × efficiency
3. Runtime
Usable energy ÷ total running watts
Checked example: A 12 V, 100 Ah lead-acid bank contains 1,200 Wh. At 50% usable capacity and 85% conversion efficiency, it provides about 510 Wh. A 100 W appliance plus 5 W inverter idle draw gives approximately 4.86 hours.
Battery runtime questions
What usable-capacity percentage should I choose?
Around 50% is conservative for many flooded lead-acid batteries, roughly 60% for AGM, and around 80–90% for many LiFePO₄ systems. Follow the battery manufacturer’s limits.
How do series and parallel batteries change the calculation?
In series, voltages add while amp-hours remain the same. In parallel, amp-hours add while voltage remains the same. Enter the final bank voltage and total bank amp-hours.
Why include inverter idle draw?
An inverter consumes power even before the appliance load is counted. Including idle draw matters most with small loads or long runtimes.
Does battery age affect runtime?
Yes. Older batteries can store less energy and may sag more under load. Reduce usable capacity when estimating an aged or cold battery bank.
Using a packaged portable power station?
Use its watt-hour rating directly with our power station calculator.