Work out how long your UPS will keep a router, NAS, desktop, or server running when the power goes out. Enter the batteries inside it — the label usually reads something like 12V 9Ah — and the load in watts or VA. The calculator builds the battery bank, applies inverter losses and a realistic depth of discharge, and shows the backup time plus a table of the same bank at common home and server loads, with an optional Peukert correction for sealed lead-acid batteries.
Everything is calculated in your browser. For batteries outside a UPS — power banks, LiFePO4 packs, or leisure batteries — use the battery runtime calculator. To convert between amps, volts, and watts, try the Ohm's law calculator, and if you are wiring an external battery pack, the wire size calculator picks a cable gauge.
Frequently asked questions
How do I calculate UPS backup time?
Multiply the battery bank voltage by its amp-hour rating to get watt-hours, take the share the UPS will actually use (depth of discharge, about 80 %), and divide by the power drawn from the battery, which is the load in watts divided by the inverter efficiency (about 85 %). A UPS with one 12 V 9 Ah battery holds 108 Wh; 80 % of that is 86 Wh, and a 60 W NAS draws about 71 W from the battery, giving roughly 1 h 13 min. The calculator does this for you and also shows the same bank at other common loads.
What is the difference between VA and watts on a UPS?
VA (volt-amperes) is apparent power; watts are the real power your equipment consumes. They differ by the power factor: W = VA × PF. A UPS labelled 1500 VA / 900 W can supply up to 900 W of real load, whichever limit you hit first. Modern computer power supplies with active power factor correction run at 0.9–0.99, so their watts and VA are close. Enter your load in watts if you measured it with a plug-in power meter, or in VA with a power factor if that is all the label gives you, and add the UPS rating to see the load percentage.
Why does my UPS run for less time than this estimate?
Manufacturer runtime charts are nonlinear. At high load the battery voltage sags quickly, lead-acid chemistry delivers less of its rated capacity (the Peukert effect), and the UPS cuts off at a low-voltage threshold well before the battery is empty. A UPS near 100 % load may run only a few minutes even when the simple calculation suggests ten or more. Batteries also age: sealed lead-acid UPS batteries typically lose 20–50 % of their capacity within three to five years, especially in warm rooms. Enable the Peukert option for a more conservative figure and use the spec-sheet chart for critical systems.
How are multiple UPS batteries wired?
Almost all UPSs wire their internal batteries in series to raise the voltage: two 12 V 9 Ah batteries make a 24 V 9 Ah bank, and four make 48 V 9 Ah. The amp-hour rating stays the same in series, but the stored energy doubles or quadruples because the voltage does. This calculator therefore treats the bank as n batteries in series; the watt-hours, and so the runtime, come out the same as a parallel arrangement with identical batteries. External battery packs for larger UPSs usually add capacity at the same bank voltage.
What load should I plan for?
Measure if you can — a plug-in power meter between the wall and the UPS gives the real figure, which is usually far below the power supply rating printed on the equipment. Typical values are 15–25 W for a modem and router, 40–80 W for a small NAS, 100–200 W for a desktop PC with monitor under light use, and 300–500 W for a gaming PC under load. Plan for the heaviest realistic load, then keep it under about 70–80 % of the UPS watt rating for headroom and a longer battery life.