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.
1 kW·h = 3,600,000 J
1 W·h = 3,600 J
1 W·s = 1 J
1 W = 1 J/s
A 60 W light bulb consumes 60 W of power. This is the same as 60 J/s or 216,000 J/h or 60 W·s per second or 60 W·h per hour.
The relationship between power (P), energy (E) and time (t) is given by the formula:
- E = P · t
If we use SI units,
- P is measured in watts
- t is measured in seconds
- E is measured in joules
- P is given in kilowatts
- t is given in hours
- E is in kilowatt hours
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 electrically supplied 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 Board of Trade Unit or B.O.T.U., defined by a former department of the UK government, is an alias. The B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy.
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.