The kilowatt-hour is a unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt-second is equal to 1 J. 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.