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 newtonmetre and, in terms of SI base units:
An energy unit that is used in atomic physics, particle physics and high energy physics is the electronvolt (eV). One eV is equivalent to 1.60217653×10^{−19} J. In spectroscopy the unit cm^{−1} = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation E = hν = hc / λ.
In discussions of energy production and consumption, the units barrel of oil equivalent and ton of oil equivalent are often used.
When discussing amounts of energy released in explosions or bolide impact events, the TNT equivalent unit is often used. 1 ton of TNT equivalent is equal to 4.2 × 10^{9} joules. Therefore, 1 kt TNT is 4.2 × 10^{12} joules, and 1 Mt TNT is 4.2 × 10^{15} joules.
Note that torque, the "rotational force" or "angular force" which causes a change in rotational motion is typically expressed in newtonmetres. This is not a simple coincidence: a torque of 1 newtonmetre applied on 1 radian requires exactly 1 newtonmetre = 1 joule of energy.
In cgs units, one erg is 1 g cm^{2} s^{−2}, equal to 1.0×10^{−7} J.
The imperial/U.S. units for both energy and work include the footpound force (1.3558 J)l, the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepowerhour (2.6845 MJ).
The energy unit used for everyday electricity, particularly for utility bills, is the kilowatthour (kWh), and one kWh is equivalent to 3.6×10^{6} J (3600 kJ or 3.6 MJ). Electricity usage is often given in units of kilowatthours per year (kWh/yr). This is actually a measurement of average power consumption, i.e., the average rate at which energy is transferred.
The calorie equals the amount of thermal energy necessary to raise the temperature of one gram of water by 1 Celsius degree, at a pressure of 1 atm. 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, often simply written capitalized as "Calories" (= 10^{3} calories).
In physics and chemistry, it is still common to measure energy on the atomic scale in the nonSI, 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 hc being the proportionality constant.
For conversion of units of energy, see energy unit conversion.
