watt
Powerexact by definition
The watt is the coherent SI unit of power, one joule per second. Its factor is 1 by construction and it is exact, being assembled from base units with no numerical constant. It is equally one newton metre per second of mechanical power and one volt-ampere of electrical power in a resistive circuit.
| 1 W | 1 W |
Named for James Watt, whose improvements to the steam engine in the 1760s and 1770s made engine output something worth quantifying in the first place. The name was adopted internationally in 1889 and confirmed by the CGPM in 1960.
About the watt
Power is the rate of doing work, and the watt's definition as one joule per second makes every energy calculation and every power calculation two sides of one arithmetic. Multiply watts by seconds and you get joules; divide joules by seconds and you get watts. The failure to keep these straight is behind most confused energy conversations, where a solar panel's rating in kilowatts is compared against a household's consumption in kilowatt-hours as though they were the same kind of number.
The watt's unification across domains is its real power. A 100 W motor, a 100 W heater and a 100 W light bulb consume energy at the same rate, though they do different things with it, and the same unit applies to a person's metabolic output (about 100 W at rest), to sunlight arriving at the ground (about 1000 W/m² at noon on a clear day), and to a power station (hundreds of megawatts). Mechanical power has an especially useful form: power equals force times velocity, and equals torque times angular velocity, so a motor's rating, its torque curve and its speed are three views of one quantity.