A watt is the basic unit of power in the International System of Units (SI). It measures the rate at which electrical energy is transferred by an electric circuit. It is also used as a measure of the amount of energy transfer required to perform work. A watt is equal to 1 joule per second. Because a watt is a relatively small amount of power, units that reflect multiples of watts are more commonly used in the energy industry including the kilowatt (1,000 watts), megawatt (1,000,000 watts), and gigawatt (1,000,000,000 watts). The watt is named after James Watt, an 18th-century Scottish inventor credited with development of the steam engine.
The kilowatt (kW) is commonly used to describe the demand put on the system by end-use customers while the megawatt (MW) is used to describe the capacity of power plants and overall system demand.