UPS / Battery Backup Runtime Calculator
How long will your UPS or power station keep things running when the grid drops? Enter the battery capacity and your load, and get a realistic runtime — losses and safe discharge limits included.
Desk UPS 100–500 Wh · power station 500–3,000 Wh · home battery 5,000+ Wh.
Router 15–30 W · PC 150–300 W · TV 60–150 W · fridge ~150 W avg.
Estimated runtime
0 h
at 100 W continuous load
Estimates only. Lead-acid batteries deliver less at high loads (Peukert effect), and capacity fades with age and heat.
How this UPS runtime calculator works
Runtime = battery Wh × usable capacity × inverter efficiency ÷ load watts. If you only know amp-hours, Wh = Ah × voltage. The usable-capacity factor stops you counting energy the battery can't safely give (especially lead-acid, which shouldn't go below 50%), and the efficiency factor covers the ~15% a small inverter burns as heat.
Runtime scales inversely with load: halve the watts, double the time (or better on lead-acid). That's why the smartest outage move is running only the router on the UPS — hours of internet instead of minutes of desktop.
UPS runtime FAQ
How do I calculate UPS runtime?
Runtime (hours) = battery Wh × DoD × inverter efficiency ÷ load watts. A 1,000 Wh unit at 80% DoD and 85% efficiency running 100 W lasts 6.8 hours.
How do I convert Ah to Wh?
Wh = Ah × voltage. Two 12 V 9 Ah batteries in series (24 V) hold 216 Wh. VA ratings describe inverter output capability, not stored energy — runtime comes from Wh.
Why does my UPS last less than the label suggests?
Lead-acid delivers less at high discharge rates (Peukert effect), capacity fades with age and heat, and small inverters run 80–85% efficient. Halving the load usually more than doubles runtime.
What can I run on a UPS during an outage?
Router + modem (15–30 W) for hours — the highest-value use. A desktop (150–300 W) gets minutes, enough to save work. Fridges and pumps need a big inverter or power station.