Worked example
A 150 Ah, 12V battery running a 400W load lasts about 1.8 hours at a safe 50% depth of discharge, or up to 3.6 hours if fully drained.
Calculate your backup time
Inverter Battery Backup Calculator
How long your battery will actually last
Safe backup time
1.8 hrs
50% depth of discharge
Full-capacity time
3.6 hrs
fully drained
The full-capacity figure assumes the battery drains completely — repeatedly deep-cycling a lead-acid battery this far shortens its life. The safe figure uses a 50% depth of discharge, a commonly recommended limit for longer battery lifespan.
How this is calculated
Watt-hours available. Battery Ah × voltage × 80% round-trip efficiency gives the usable watt-hours, accounting for inverter conversion and battery losses.
Divide by load. Backup hours = usable watt-hours ÷ connected load in watts. The safe figure applies a 50% depth-of- discharge limit on top of that.
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Frequently asked questions
Why does the calculator show two different backup times?
The full-capacity figure is the theoretical maximum if you drain the battery completely. The safe figure uses a 50% depth of discharge, which is a widely recommended limit for lead-acid batteries — draining deeper repeatedly shortens the battery's usable life significantly.
How do I find my battery's Ah rating?
It's printed on the battery's nameplate or case, usually alongside the voltage — for example "12V 150Ah".
Does battery age affect real backup time?
Yes, significantly. A battery's usable capacity degrades with age and charge cycles — an older battery may deliver noticeably less than its rated Ah. This calculator uses the nameplate rating, which reflects a new, fully healthy battery.
What load should I enter?
Add up the wattage of everything actually running on the inverter during the cut — not your inverter's VA rating. If you're unsure of your total load, see our inverter sizing calculator.