Published on Mar 12 2024

The watt (W) is the unit of power in the International System of Units (SI), representing 1 joule per second or 1 kg⋅m²⋅s⁻³. Named after James Watt, it measures the rate of energy transfer and played a key role in the Industrial Revolution.


When maintaining an object's velocity at 1 meter per second against a 1-newton opposing force, the work rate is 1 watt.

1 W = 1 J/s = 1 N⋅m/s = 1 kg⋅m²⋅s⁻³.

In electromagnetism, 1 watt equals the rate of electrical work with 1 ampere flowing across a potential difference of 1 volt.

1 W=1 V⋅1 A.


  • • Climbing a 3-meter-high ladder in 5 seconds, a person with a mass of 100 kg works at about 600 watts.
  • • A laborer sustains an average output of around 75 watts over an eight-hour day.

Origin and Adoption as an SI Unit

Named by C. William Siemens in 1882, the watt was internationally defined in 1908. Initially tied to practical units, it was later redefined in 1948 and fully adopted into the SI in 1960.


Examples of multiples include kilowatts, megawatts, gigawatts, terawatts, petawatts, and yottawatts, each representing varying power scales.

Conventions in the Electric Power Industry

In the electric power industry, distinctions are made between electrical and thermal power. Megawatt electrical (MWe) refers to electric power, while megawatt thermal (MWt) refers to thermal power. The SI prefix system is used for larger units, like gigawatt electrical (GWe).

Radio Transmission

Radio stations report transmitter power in watts, specifically effective radiated power, representing the power needed for a half-wave dipole antenna to match the intensity of the transmitter's main lobe.

Distinction between Watts and Watt-hours

Power and energy are related but distinct. Power (watts) is the rate of energy generation or consumption, while energy is measured in watt-hours. For instance, a 100W light bulb used for an hour consumes 100 watt-hours.