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Power and watts

Why a kettle costs far more to run than a phone charger, and what a kilowatt-hour is.

2 min read Last updated 3 Nov 2026
A kettle and a phone charger side by side, with a tall bar over the kettle and a short bar over the charger showing how much more power the kettle uses.

The short version

Power, measured in watts, is how fast energy is being used. For electricity it equals voltage multiplied by current. Your bill is charged in kilowatt-hours: the energy used by a 1,000 watt appliance running for one hour. It builds on what electricity is.

I always assumed watts and kilowatt-hours were the same thing.

They are not. One is a speed. The other is a distance.

Watts are how fast. Kilowatt-hours are how much.

Watts: how fast energy is used

A watt is one joule of energy per second. For electricity, power is the push multiplied by the flow: P = V × I.

On a 230 V supply, a 13-amp plug can carry 230 × 13 = about 3,000 watts. That is why the biggest everyday appliances, such as kettles, are rated at up to about 3 kilowatts. A kilowatt (kW) is 1,000 watts.

Kilowatt-hours: how much is used

Energy used is power multiplied by time. A 3 kW kettle running for one hour would use 3 kilowatt-hours (kWh). A 10 W bulb running for 100 hours would use 1 kWh.

A kettle takes only about three minutes to boil, which is 0.05 of an hour, so one boil uses about 0.15 kWh. If a unit costs 25p, that is under 4p.

Kilowatt-hour (kWh)

The energy used by a 1,000 watt appliance in one hour. Your electricity bill calls this a unit.

A phone charger left plugged in uses as much electricity as a kettle.

A charger with nothing attached draws a tiny amount, well under one watt. A kettle, while it is on, draws about 3,000 watts. Over a year, though, many small standby loads do add up.

Sources: Ofgem: energy bills, unit rates and standing charges; standard physics references. The 25p unit rate is a worked example only; actual tariffs vary.

Key takeaways

  • A watt is a rate of using energy.
  • P = V × I.
  • A kilowatt-hour is power times time.
  • Bills are charged by the kilowatt-hour.

Now you know.

Watts are how fast.

Kilowatt-hours are how much.

And your bill counts the second.

Last reviewed 3 Nov 2026

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