1.3 Energy units
Energy is the capacity to do work but to fully understand the topic it is important to understand the various ways it is measured.
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Joule (J).This is the basic energy unit of the metric system, or in a later more comprehensive formulation, the International System of Units (SI). It is ultimately defined in terms of the meter, kilogram, and second.
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Calorie (cal). Historically the calorie was defined in terms of the heating of water. Thus, in a traditional definition, one calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C, from 14.5 °C to 15.5 °C.
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British Thermal Unit (Btu).This is the English system analog of the calorie. For specific heat capacities to be the same, whether expressed in Btu/lb-°F or in cal/gm-°C:
1 Btu = 251.9958 cal.
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Kilowatt-Hour (kWh).The kilowatt-hour is a standard unit of electricity production and consumption. By definition, noting that 1 kilowatt = 1000 watts:
1 kWh = 3.6 x 10 6 J (exact) = 3412 Btu.
(Source : https://www.aps.org/policy/reports/popa-reports/energy/units.cfm)
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Barrel Of Oil Equivalent (BOE) is a term used to summarize the amount of energy that is equivalent to the amount of energy found in a barrel of crude oil.This is also known as crude oil equivalent (COE). BOE is frequently used when exploration and production companies are reporting the total amount of reserves they have.
1 BOE = 5.8 MBtu. = 1700 kWh.
(Source : http://www.investopedia.com/terms/b/barrelofoilequivalent.asp)
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Tonne of Oil Equivalent (TOE) is a unit representing energy generated by burning one ton (1000 kilograms or 2204.68 pounds) or 7.4 barrels of crude oil.
1 TOE = the energy obtained from 1270 cubic meters of natural gas
= the energy obtained from1.4 metric tons of coal
= 41.87 gigajoules (GJ) = 39.68 million Btu (MMBtu) =11.63 megawatt hours (MWh).
(Source : http://www.businessdictionary.com/definition/tonne-of-oil-equivalent-TOE.html)