Battery Runtime Calculator

Battery Runtime - figures update as you type, nothing leaves your browser.

Maintained by Roshan.

Use Battery Runtime Calculator

Figures update as you type. Nothing you enter leaves this tab, and no values are stored.

Battery capacity (Wh)Load power (W)Inverter efficiency (%)

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About Battery Runtime Calculator

A battery runtime calculator estimates how long a battery will power a given load, from its capacity in watt-hours, the load in watts, and the inverter's efficiency. It answers the practical question behind any backup, power station or UPS purchase: will this actually keep my things running long enough? Enter the capacity, the load and the efficiency, and the runtime in hours and minutes appears instantly, with a note that heavy loads shorten it. Every portable power station, UPS and off-grid battery bank comes down to one question this answers: how long will it actually run my load? The maths is capacity divided by load, adjusted for the inverter's efficiency, since converting stored DC to usable AC always loses a little. The tool is deliberately honest that its answer is optimistic. A battery's rated capacity is not all usable - discharging many chemistries fully shortens their life - and real runtime falls under surging loads, in the cold, and as a pack ages and loses capacity. Treat the figure as a ceiling, size up for anything you truly depend on, and for a UPS make sure it buys enough time to shut things down cleanly.

Using Battery Runtime Calculator

Enter the battery capacity in watt-hours (Wh).
Enter the load - the total watts of what you are powering.
Enter the inverter efficiency (often around 85–90%), and see the estimated runtime.

Where battery runtime calculator helps

Sizing a power station

A 1000Wh station at 90% efficiency powering a 200W load lasts about 4.5 hours.

Planning a UPS

Check whether a UPS will keep essential gear up long enough to shut down or ride out a cut.

Account for changing loads

A 60 W lamp and a 140 W computer together draw 200 W, so use the combined demand for an estimate of simultaneous operation. If the computer later idles, that constant-load estimate no longer describes the whole session. The displayed duration is an energy calculation; it does not establish whether the battery inverter can supply a short startup surge.

Before you use the result

It is an estimate: real runtime is shorter under surging loads, at low temperatures, and as a battery ages and loses capacity.
Do not discharge many battery types fully; usable capacity is often less than the rated figure.
Nothing entered is stored beyond this tab.

Questions about Battery Runtime Calculator

How long will my battery last?

Divide usable capacity (capacity times efficiency) by the load in watts. The tool does it and warns real runtime is lower.

Why is real runtime shorter?

Surging loads, cold temperatures and battery ageing all reduce it below the ideal figure.