A watt is a unit of power. Power is the rate at which energy is used, and a watt is equal to a rate of one joule of energy per second. Watts are commonly used when referring to the energy consumption of relatively small things like lightbulbs, while kilowatts (a thousand watts) are used for larger machines. Megawatts (a million watts) are used to measure the electricity generation of power stations. See also kilowatt-hours.