UPS / Battery Backup Runtime Calculator
How long will your UPS or power station keep things running when the grid drops? This UPS battery runtime calculator takes your battery capacity (Ah or VA) and your load in watts and returns a realistic backup time — losses and safe discharge limits included. Works for online, line-interactive, and offline UPS units and DIY battery banks.
Reviewed by MyEnergyToolkit Editorial Team · Updated July 23, 2026 · Methodology & sources
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.