Energy is defined viawork, so theSIunit of energy is the same as the unit of work – thejoule (J), named in honour ofJames Prescott Joule^[1] and his experiments on themechanical equivalent of heat. In slightly more fundamental terms,1 joule is equal to 1 newton metre and, in terms ofSI base units

${\displaystyle 1\mathrm{J}=1\mathrm{k}\mathrm{g}{\left(\frac{\mathrm{m}}{\mathrm{s}}\right)}^2=1\frac{\mathrm{k}\mathrm{g}\cdot {\mathrm{m}}^2}{{\mathrm{s}}^2}}$

An energy unit that is used inatomic physics,particle physics, andhigh energy physics is theelectronvolt (eV). One eV is equivalent to1.602176634×10⁻¹⁹ J.^[2]

Inspectroscopy, the unit cm⁻¹ ≈0.0001239842 eV is used to represent energy since energy is inversely proportional to wavelength from the equation${\displaystyle E=h\nu =hc/\lambda }$.

In discussions of energy production and consumption, the unitsbarrel of oil equivalent andton of oil equivalent are often used.

TheBritish imperial units andU.S. customary units for both energy and work include thefoot-pound force (1.3558 J), theBritish thermal unit (BTU) which has various values in the region of 1055 J, thehorsepower-hour (2.6845 MJ), and thegasoline gallon equivalent (about 120 MJ).

The table illustrates the wide range of magnitudes among conventional units of energy. For example, 1 BTU is equivalent to about 1,000 joules, and there are 25 orders-of-magnitude difference between a kilowatt-hour and an electron-volt.

A unit of electrical energy, particularly for utility bills, is thekilowatt-hour (kWh);^[3] one kilowatt-hour is equivalent to3.6 megajoules. Electricity usage is often given in units of kilowatt-hours per year or other periods.^[4] This is a measurement of averagepower consumption, meaning the average rate at which energy is transferred. One kilowatt-hour per year is around 0.11 watts.

Natural gas is often sold in units of energy content or by volume. Common units for selling by energy content arejoules ortherms. 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). In Australia, natural gas is sold in cubic metres. One cubic metre contains about 38 megajoules. In most of the world, natural gas is sold in gigajoules.

Thecalorie is defined as the amount ofthermal energy necessary to raise thetemperature of onegram of water by 1 Celsius degree, from a temperature of14.5 °C, at apressure of1 atm. For thermochemistry a calorie of4.184 J is used, but other calories have also been defined, such as the International Steam Table calorie of4.1868 J. In many regions,food energy is measured inlarge calories (alarge calory is a kilocalory, equal to1000 calories), sometimes written capitalized asCalories. In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information.

In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, unitselectronvolts (eV). 1 eV is equivalent to the kinetic energy acquired by an electron in passing through a potential difference of 1 volt in a vacuum. It is common to use the SI magnitude prefixes (e.g. milli-, mega- etc) with electronvolts. Because of therelativistic equivalence between mass and energy, the eV is also sometimes used as a unit of mass. TheHartree (theatomic unit of energy) is commonly used in the field of computational chemistry since such units arise directly from the calculation algorithms without any need for conversion. HistoricallyRydberg units have been used.

Inspectroscopy and related fields it is common to measure energy levels in units ofreciprocal centimetres. These units (cm⁻¹) are strictly speaking not energy units but units proportional to energies, with${\displaystyle hc\sim 2\cdot {10}^{-23}\mathrm{J}\mathrm{c}\mathrm{m}}$  being the proportionality constant.^[5]

A gram ofTNT releases 4,100 to 4,600joules (980 to 1,100calories) upon explosion. To define thetonne 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.^[6]

• Energy consumption
• Conversion of units of temperature
• Conversion of units of energy, work, or amount of heat
• Kilokaiser
• List of unusual units of measurement
• Maximum demand indicator
• Orders of magnitude (energy)
• erg
• Foe (unit)