Mobile & Electronics

Battery Runtime Calculator

Runtime straight from a battery's watt-hours - for laptop batteries and anything else labelled in energy.

7 inputs Free, no sign-up

Your figures

The Wh figure on the battery. Laptop batteries and anything shipped by air carry it.

What the battery has to run, in watts. It is on the rating plate of whatever you are powering.

Leave blank for this page's 90% assumption.

Only needed if you want the capacity in Ah shown alongside.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

When a battery is labelled in watt-hours, the runtime calculation skips a step. Energy divided by load gives time directly — no voltage, no conversion from amp-hours.

That is why laptop batteries are labelled that way. A 56 Wh battery running a 20 W load lasts about two and a half hours, and the arithmetic is one division.

How to use this calculator

  1. Enter the battery energy in watt-hours, from its label.
  2. Enter the load in watts.
  3. Set an efficiency if the energy passes through a converter. For a laptop running from its own battery, losses are small.
  4. Add the voltage only if you want to see the capacity in amp-hours as well.

The formula

Usable energy = Battery energy x Efficiency
Runtime       = Usable energy / Load

If your battery is labelled in mAh or Ah instead, convert it first with the mAh to Wh calculator, or use the battery backup calculator, which takes voltage and capacity directly. All three run on the same engine, so they agree.

SymbolMeaningUnit
Wh Battery energy Wh
Load Power being drawn W
Efficiency Share reaching the load %

Worked example

A 56 Wh laptop battery running a 20 W load at 90%:

Usable  = 56 x 0.90  = 50.4 Wh
Runtime = 50.4 / 20  = 2.52 hours = 2 h 31 min

Notes and limits

  • An estimate. A laptop's draw varies constantly with what it is doing, so a single load figure is an average at best.
  • Screen brightness is usually the largest single lever on a laptop's draw, followed by whether the processor is working hard.
  • Batteries lose capacity with age. A three-year-old 56 Wh battery may hold 45 Wh, and using the label figure will overstate the runtime accordingly.
  • Efficiency near 100% is right for a load running directly off the battery. Lower it only where the energy passes through a converter.

What this calculation assumes

  • The load is steady at the figure entered, which for a laptop is an average at best.
  • The battery holds its labelled energy, which an older one will not.
  • Efficiency is your figure or this page's stated assumption.

Frequently asked questions

How long will a 56 Wh laptop battery last?

Divide by the load. At 20 W it is about 2.5 hours; at 10 W, around 5. The load depends on screen brightness and what the machine is doing, and varies constantly in practice.

How do I find my laptop's power draw?

Battery status tools on most operating systems report current draw in watts. Failing that, work backwards: if a full battery lasts four hours, the average draw was the battery energy divided by four.

Why does my laptop last less than this suggests?

Usually because the average draw is higher than the figure entered, or because the battery has aged and no longer holds its rated energy. Both are common and they compound.

Should I use watt-hours or amp-hours?

Watt-hours where the label gives them, since no voltage is needed. Amp-hours need the voltage too, and the battery backup calculator takes that pair.

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