Computers & Laptops

Laptop Battery Runtime Calculator

How long a laptop battery lasts, from its watt-hours and the power the machine is actually drawing.

7 inputs Free, no sign-up

Your figures

The Wh figure printed on the battery or in the machine's specification. Laptop batteries are always labelled this way, because watt-hours is the figure that actually compares.

What the machine uses while doing the work you have in mind. Idle with the screen dim and a compile under load are several times apart, so the honest answer is a range.

Leave blank for this page's 90% assumption, which the result names. A little of the battery's energy is lost in the machine's own voltage regulation.

Optional, and 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

Battery life is energy divided by load, and the load is the part nobody knows. A 56 Wh battery lasts seven hours at 8 W and under two hours at 30 W, and both of those are ordinary figures for the same machine doing different work.

So this asks for the draw rather than assuming one, and says plainly that the answer is an estimate.

How to use this calculator

  1. Find the battery's watt-hour figure — on the battery itself, in the machine's specification, or in your operating system's battery report.
  2. Enter the power the machine draws doing the work you care about. Your operating system's power tools will report it, or work it back from a session you have timed.
  3. Leave the efficiency blank unless you have a better figure.

The formula

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

The efficiency covers what is lost between the cells and the components. It is an assumption, and the result names whichever figure it used.

SymbolMeaningUnit
Battery energy Capacity in watt-hours Wh
Load What the machine draws W
Efficiency Share reaching the load %

Worked example

A 56 Wh battery at 90% efficiency, driving a 12 W load:

Usable  = 56 x 0.90 = 50.4 Wh
Runtime = 50.4 / 12  = 4.2 hours

The same battery at 30 W gives about 1.7 hours.

Notes and limits

  • Batteries lose capacity with age. A three-year-old laptop battery commonly holds 80% of what it did new, and your operating system's battery report will give the figure.
  • Screen brightness is one of the largest single draws on a laptop, and often the easiest to change.
  • Manufacturers' battery-life claims come from a light, standardised workload. They are not wrong, but they are not your workload.
  • To measure the load rather than guess it: note the battery percentage, work for an hour, and the drop multiplied by the battery's watt-hours is the average draw.

What this calculation assumes

  • The load is treated as constant, which it never quite is.
  • The battery is treated as holding its rated capacity, which an older one does not.
  • Efficiency defaults to 90% and the result names it as an assumption.

Frequently asked questions

How long will my laptop battery last?

Divide the battery's watt-hours by the power the machine is drawing, allowing for a little loss. A 56 Wh battery at 12 W gives about four hours; the same battery under heavy load gives well under two.

Where do I find my battery's Wh figure?

On the battery, in the machine's specification, or from a battery report - `powercfg /batteryreport` on Windows, System Information on macOS, or `upower -i` on Linux. The report also shows how much capacity the battery has lost.

Why is my real battery life shorter than this?

Usually because the average load is higher than the figure entered, or the battery has aged. Both are worth measuring: the load from a timed session, the capacity from a battery report.

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