Definition. The kilowatthour is a composite unit of energy equal to one kilowatt (kW) sustained for (multiplied by) one hour. Expressed in the standard unit of energy in the International System of Units (SI), the joule (symbol J), it is equal to 3,600 kilojoules or MJ.. Unit representations. A widely used representation of the kilowatthour is "kWh", derived from its component units ...