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 electric appliance operating continuously for 25 hours uses one kilowatt hour. In terms of human power, a healthy adult male manual laborer will perform work equal to about half a kilowatt hour over an eight hour day.
Electrical energy is often sold in kilowatt hours. The cost of running an electric device is calculated by multiplying the device's power in kilowatts, by the running time in hours, by the 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.
Whereas individual homes only pay for the kilowatt hours consumed, commercial buildings and institutions also pay for peak power consumption, the greatest power recorded in a fairly short time, such as 15 minutes. This compensates the power company for maintaining the infrastructure needed to provide peak power. These charges are billed as demand charges.