UPS Battery Runtime Calculator

Runtime in minutes is battery watt-hours times efficiency, divided by the load in watts, times 60. Hours is that figure divided by 60. So 1000 Wh at 0.9 efficiency and a 300 W load gives (1000 x 0.9 / 300) x 60 = 180 minutes.

Multiply the battery voltage by its amp-hour rating. A 12 volt 100 Ah battery holds 12 x 100 = 1200 watt-hours. Switch the tool to Volts and amp-hours mode and it does this step for you before estimating runtime.

Around 0.9 is typical for a UPS inverter converting DC battery power to AC. Older, cheaper, or heavily loaded units may sit closer to 0.8, which shortens the runtime. The field accepts any value above 0 and up to 1.

Lead-acid batteries deliver less capacity at high discharge rates, an effect described by Peukert's law, and you should avoid draining them fully to protect their lifespan. Battery age, temperature, and the UPS overhead all reduce real runtime, so treat this number as an optimistic upper bound.

Roughly, yes. Runtime is inversely proportional to the load in this model, so moving from a 150 W load to 300 W on the same battery cuts the estimate in about half. In practice the drop can be slightly steeper because of Peukert losses at higher current.

The battery energy is rounded to four decimals, the minutes to two decimals, and the hours to four decimals internally, though the display trims trailing digits. An 800 Wh battery at 350 W and 0.9 efficiency shows 123.43 minutes, about 2.06 hours.

Yes. Find the device wattage from its label or a plug-in power meter, enter it as the load, and enter your battery or UPS rating. The result tells you roughly how many minutes that device would keep running if utility power failed.

VA is apparent power, not the battery energy this tool needs. Many small UPS units publish an internal battery like 12 V 7 Ah or 9 Ah. Enter that in Volts and amp-hours mode, and use the real device wattage, not the UPS VA number, as the load.

No. The calculation runs entirely in your browser using JavaScript, so the battery ratings and loads you type never leave your device and nothing is stored on a server.