Watt

The watt is named after James Watt for his contributions to the development of the steam engine. The unit was recognized by the Second Congress of the British Association for the Advancement of Science in 1889. The 11th General Conference on Weights and Measures in 1960 adopted it for the measurement of power into the International System of Units (SI).

One kilowatt of power is very close to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average annual electrical energy consumption of a household in the United States is about 8,900 kilowatt-hours, equivalent to a steady power consumption of about 1 kW, for an entire year, Also, kilowatts of light power can be measured in the output pulses of some lasers.

The productive capacity of electrical generators operated by a utility company is often measured in MW. On railways, modern high-powered electric locomotives typically have a peak power output of 5 or 6 MW although some produce much more—the Eurostar, for example, consumes more than 12 MW—while heavy diesel-electric locomotives typically consume 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.

In the electric power industry, megawatt electrical (abbreviation: MWe or MWe) is a term that refers to electric power, while megawatt thermal or thermal megawatt (abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe).[notes 1]

Power and energy are frequently confused. Power is the rate at which energy is generated and consumed.

Terms such as watts per hour are often misused. Watts per hour properly refers to the change of power per hour. Watts per hour (W/h) might be useful to characterize the ramp-up behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations.

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