In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V), meaning the watt is equivalent to the volt-ampere (the latter unit, however, is used for a different quantity from the real power of an electrical circuit).
Two additional unit conversions for watt can be found using the above equation and Ohm's law.
[5] The unit name was proposed by C. William Siemens in August 1882 in his President's Address to the Fifty-Second Congress of the British Association for the Advancement of Science.
[8] In October 1908, at the International Conference on Electric Units and Standards in London,[9] so-called international definitions were established for practical electrical units.
After the 9th General Conference on Weights and Measures in 1948, the international watt was redefined from practical units to absolute units (i.e., using only length, mass, and time).
Concretely, this meant that 1 watt was defined as the quantity of energy transferred in a unit of time, namely 1 J/s.
[11] In the electric power industry, megawatt electrical (MWe[31] or MWe)[32] refers by convention to the electric power produced by a generator, while megawatt thermal or thermal megawatt[33] (MWt, MWt, or MWth, MWth) refers to thermal power produced by the plant.
For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2,109 MWt (i.e. heat), which creates steam to drive a turbine, which generates 648 MWe (i.e. electricity).
[35] When describing alternating current (AC) electricity, another distinction is made between the watt and the volt-ampere.
While these units are equivalent for simple resistive circuits, they differ when loads exhibit electrical reactance.
This refers to the power that a half-wave dipole antenna would need to radiate to match the intensity of the transmitter's main lobe.
The terms power and energy are closely related but distinct physical quantities.
Power stations are rated using units of power, typically megawatts or gigawatts (for example, the Three Gorges Dam in China is rated at approximately 22 gigawatts).
This reflects the maximum power output it can achieve at any point in time.
One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year: equivalent to approximately 114 megawatts of constant power output.