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Because energy is defined via work the SI unit for energy is the same as the unit of work – the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton-metre and, in terms of SI base units:

1\ \mathrm{J} = 1\ \mathrm{kg} \left( \frac{\mathrm{m}}{\mathrm{s}} \right ) ^ 2 = 1\ \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}

Other units of energyEdit

In cgs units, one erg is 1 g cm2 s−2, equal to 1.0×10−7 J.

The imperial/U.S. units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepower-hour (2.6845 MJ).

The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×106 J  (3600 kJ or 3.6 MJ).

The calorie equals the amount of heat necessary to raise the temperature of one gram of water by 1 Celsius degree, at a pressure of 1 atmospheric pressure. For thermochemistry a calorie of 4.184 J is used, but other calories have also been defined, such as the International Steam Table calorie of 4.1868 J. Food energy is measured in large calories or kilocalories, sometimes capitalized as "Calories" (= 103 small calories).

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