In electricity and magnetism, the watt is the SI unit that measures electric power.
One watt equals one joule of energy transferred or converted per second. In an electrical circuit, power can be calculated as voltage multiplied by current, so P = VI. For example, a device operating at 10 volts and drawing 2 amperes uses 20 watts.
The unit is named after Scottish engineer James Watt, whose work improving steam engines helped popularize the measurement of mechanical power. The watt became part of the International System of Units in the 20th century.
A watt measures the rate of energy transfer, not the total amount of energy. The joule measures energy, while the volt measures electric potential difference and the ohm measures resistance. Larger powers are commonly expressed in kilowatts, megawatts, or gigawatts.