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.

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.
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.
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.
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 Links to an external site.)
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 Links to an external site. 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 Links to an external site.)
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 Links to an external site.)