Mobile & Electronics

Electrical Energy Calculator

Watt-hours and units from a power and a running time, in whatever units you have them.

4 inputs Free, no sign-up

Your figures

From the rating plate or the adapter.

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

Energy is power multiplied by time. A 65 W laptop running for eight hours uses 520 watt-hours, which is 0.52 of a unit on an electricity bill.

Power is a rate — how fast energy is being used. Energy is the total. Keeping the two apart is what makes the difference between a 2,000 W kettle (high power, on for two minutes) and a 5 W fridge light (low power, on all day) obvious.

How to use this calculator

  1. Enter the power in watts or kilowatts.
  2. Enter how long it runs, choosing minutes, hours or days — the units are converted for you, which matters because reading minutes as hours is an error of sixty.
  3. Read the energy in watt-hours and in units.
  4. For what it costs, take the figure to the electricity cost calculator.

The formula

Energy = Power x Time            E = P x t
Watt-hours = Watts x Hours
Units (kWh) = Watt-hours / 1000

A "unit" on an Indian electricity bill is one kilowatt-hour: a thousand watts for one hour, or one watt for a thousand hours, or any other combination that multiplies out the same.

SymbolMeaningUnit
P Power W
t Running time h
E Energy used Wh

Worked example

A 65 W laptop for 8 hours:

E = 65 x 8 = 520 Wh = 0.52 units

A 2,000 W kettle for 3 minutes:

3 min = 0.05 h
E = 2000 x 0.05 = 100 Wh = 0.1 units

The kettle draws thirty times the power and uses a fifth of the energy, because it is on for a fraction of the time.

Notes and limits

  • Power is a rate; energy is a total. A high-power device used briefly can cost less than a low-power one left on.
  • This is the energy the device itself uses. Losses in an adapter or an inverter between the socket and the device are additional.
  • A device's rating is usually its maximum. A laptop rated at 65 W draws that much only under load, and much less while idle.
  • Devices on standby still use energy. Where that matters, use the standby wattage and 24 hours a day rather than assuming it is nothing.

What this calculation assumes

  • The device draws the power entered for the whole time; a varying load draws more at its peaks.
  • Losses in an adapter or an inverter between the socket and the device are not included.
  • One unit on a bill is one kilowatt-hour.

Frequently asked questions

What is the difference between watts and watt-hours?

Watts measure how fast energy is used; watt-hours measure how much was used in total. A 100 W bulb uses 100 watt-hours in an hour and 200 in two hours - the wattage does not change, the energy accumulates.

What is a unit of electricity?

One kilowatt-hour, which is 1,000 watt-hours. It is what an electricity meter counts and what a tariff is priced per.

How do I convert watt-hours to kilowatt-hours?

Divide by 1,000. So 520 watt-hours is 0.52 kilowatt-hours, or a little over half a unit.

Does a device on standby use energy?

Yes, usually a few watts. It is small per hour but runs every hour of every day, which is why a handful of devices on standby can add up to a noticeable share of a bill.

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