Units of power

Beginner

Watt equals joule per second; horsepower ≈ 746 W.

Key relation1 W = 1 J/s

1 watt = 1 J/s. A horsepower is about 746 W. Human sustained output is about 100 W.

Worked example

A 2 kW kettle uses 2 kJ per second.

Common mistake

kW is power, kWh is energy.

In more detail

The watt is the SI unit of power, equal to one joule per second. Larger quantities use kilowatts, megawatts and gigawatts.

Solar panels are rated in watts of peak output.
Power stations are rated in megawatts or gigawatts.
Light bulbs, kettles and heaters are rated in watts, which tells you the energy they use each second.

How do I convert watts to kilowatt hours?

Multiply the power in kilowatts by the time in hours to get energy in kilowatt hours.

Is a watt the same as a watt hour?

No. A watt is power, while a watt hour is energy.

What is a kilowatt-hour?

An energy unit equal to 1000 watts used for one hour, which is 3.6 million joules.