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The kilowatt-hour (symbol: kW·h) is a unit for measuring energy. It corresponds to one kilowatt (kW) of power being used over a period of one hour. Since a watt is an amount of energy over a timespan (joule per second), multiplying it with a timespan gives an energy amount (1 J/s × 1 s = 1 J).
The kilowatt-hour is commonly used for electrical energy, since it may be easier to understand in a practical context than the proper SI unit for energy, the joule, which is a watt-second (W·s). The joule is a comparatively small unit, making numbers quite large.
Identities
1 kW·h = 3,600,000 J. 1 W·h = 3,600 J. 1 W·s = 1 J.
1 W·s = 1 J. 1 W = 1 J/s.
A 60 W light bulb consumes 60 W, 60 J/s, 216,000 J/h, or 60 W·h per hour.
Usage in commerce
The kilowatt-hour is a commonly used unit of billing for electric utility companies, although many companies are moving to use the unit megajoule(MJ) instead.
As such, a kilowatt-hour meter (electricity meter) is installed in most buildings in the world.
In the UK, the kilowatt-hour is commonly referred to by consumers and electricity retailers as the unit (as in "we were billed for 300 units of electricity").
The kilowatt-hour is sometimes used for billing of natural gas supply. As most gas meters measure the volume of gas supplied, the calorific value (energy content) of the gas must be accounted for in the conversion to kilowatt-hours.
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