Kilowatt-hours

The kilowatt-hour (symbol kWh,kW·h or kW h) is a derived unit of energy equal to 3.6 megajoules. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt-hours is the power in kilowatts multiplied by the time in hours. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.

The kilowatt-hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour.

One watt is equal to 1 J/s. One kilowatt-hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.

The base unit of energy within the International System of Units (SI) is the joule. The hour is a unit of time "outside the SI", making the kilowatt-hour a non-SI unit of energy. The kilowatt-hour is not listed among the non-SI units accepted by the BIPM for use with the SI, although the hour, from which the kilowatt-hour is derived, is.

An electric heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt-hour (equivalent to 3.6 megajoules) of energy. A television rated at 100 watts operating for 10 hours continuously uses one kilowatt-hour. A 40-watt light bulb operating continuously for 25 hours uses one kilowatt-hour.

Electrical energy is sold in kilowatt-hours; cost of running equipment is the product of power in kilowatts multiplied by running time in hours and price per kilowatt-hour. The unit price of electricity may depend upon the rate of consumption and the time of day. Industrial users may also have extra charges according to their peak usage and the power factor.

The symbol "kWh" is commonly used in commercial, educational, scientific and media publications, and is the usual practice in electrical power engineering.

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