As energy is defined via work, the SI unit of energy is the same as the unit of work – the joule (J), named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, is equal to 1 newton metre and, in terms of SI base units
In spectroscopy the unit cm-1 ? is used to represent energy since energy is inversely proportional to wavelength from the equation .
The British imperial units and U.S. customary 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, the horsepower-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ).
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An energy unit used for electrical power, particularly for utility bills, is the kilowatt-hour (kWh); one kilowatt-hour is equivalent to . Electricity usage is often given in units of kilowatt-hours per year or other time period. This is actually a measurement of average power consumption, meaning the average rate at which energy is transferred. One kilowatt-hour per year is about 0.11 watts.
Natural gas is often sold in units of energy content or by volume. Common units for selling by energy content are joules or therms. One therm is equal to about 1,055 megajoules. Common units for selling by volume are cubic metre or cubic feet. Natural gas in the US is sold in therms or 100 cubic feet (100 ft3 = 1 Ccf). In Australia, natural gas is sold in cubic metres. One cubic metre contains about 38 megajoules. In the most of the world, natural gas is sold in gigajoules.
The calorie is defined as the amount of thermal energy necessary to raise the temperature of one gram of water by 1 Celsius degree, from a temperature of , at a pressure of . For thermochemistry a calorie of is used, but other calories have also been defined, such as the International Steam Table calorie of . In many regions, food energy is measured in large calories or kilocalories equalling , sometimes written capitalized as Calories. In the European Union, food energy labeling in joule is mandatory, often with calories as supplementary information.
In physics and chemistry, it is still common to measure energy on the atomic scale in the non-SI, but convenient, units electronvolts (eV). The Hartree (the atomic unit of energy) is commonly used in calculations. Historically Rydberg units have been used.
In spectroscopy and related fields it is common to measure energy levels in units of reciprocal centimetres. These units (cm-1) are strictly speaking not energy units but units proportional to energies, with being the proportionality constant.
A gram of TNT releases 4,100 to 4,600 joules (980 to 1,100 calories) upon explosion. To define the tonne of TNT, this was standardized to 1 kilocalorie (4,184 joules) giving a value of 4.184 gigajoules (1 billion calories) for the tonne of TNT.