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As energy is defined via work, the SI **unit of 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 newton metre and, in terms of SI base units

- British imperial / US customary units
- Electricity
- Natural gas
- Food industry
- Atom physics and chemistry
- Spectroscopy
- Explosions
- See also
- References

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} ≈ 0.0001239842 eV is used to represent energy since energy is inversely proportional to wavelength from the equation .

In discussions of energy production and consumption, the units barrel of oil equivalent and ton of oil equivalent are often used.

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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A unit of electrical energy, particularly for utility bills, is the kilowatt-hour (kWh); one kilowatt-hour is equivalent to 3.6 megajoule. 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 ft^{3} = 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 14.5 °C, 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. In many regions, food energy is measured in large calories or kilocalories equalling 1000 calories, sometimes written capitalized as *Calories*. 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, units electronvolts (eV). Because of the relativistic equivalence between mass and energy, the eV is also sometimes used as a unit of mass. The Hartree (the atomic unit of energy) is commonly used in calculations.^{[ clarification needed ]} 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.^{ [1] }

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.^{ [2] }

The **British thermal unit** is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. Heat is now known to be equivalent to energy. The modern SI unit for heat and energy is the joule (J); One BTU equals about 1055J.

The **centimetre–gram–second system of units** is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.

In physics, an **electronvolt** is the amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. When used as a unit of energy, the numerical value of 1 eV in joules is equivalent to the numerical value of the charge of an electron in coulombs. Under the 2019 redefinition of the SI base units, this sets 1 eV equal to the exact value 1.602176634×10^{−19} J.

In physics, **energy** is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton.

The **joule** is a derived unit of energy in the International System of Units. It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of the force's motion through a distance of one metre. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).

In physics, **power** is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, power is sometimes called *activity*. Power is a scalar quantity.

A **metric prefix** is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix *kilo-*, for example, may be added to *gram* to indicate *multiplication* by one thousand: one kilogram is equal to one thousand grams. The prefix *milli-*, likewise, may be added to *metre* to indicate *division* by one thousand; one millimetre is equal to one thousandth of a metre.

In thermodynamics, the **specific heat capacity** of a substance is the heat capacity of a sample of the substance divided by the mass of the sample. Informally, it is the amount of energy that must be added, in the form of heat, to one unit of mass of the substance in order to cause an increase of one unit in temperature. The SI unit of specific heat is joule per kelvin and kilogram, J·kg^{−1}·K^{−1}. For example, the heat required to raise the temperature of 1 kg of water by 1 K is 4184 joules so the specific heat capacity of water is 4184 J·kg^{−1}·K^{−1}.

The **Boltzmann constant** is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, and in Planck's law of black-body radiation and Boltzmann's entropy formula. The Boltzmann constant has dimensions of energy divided by temperature, the same as entropy. It is named after the Austrian scientist Ludwig Boltzmann.

The **gas constant** is denoted by the symbol *R* or *R*. It is equivalent to the Boltzmann constant, but expressed in units of energy per temperature increment per *mole*, i.e. the pressure–volume product, rather than energy per temperature increment per *particle*. The constant is also a combination of the constants from Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. It is a physical constant that is featured in many fundamental equations in the physical sciences, such as the ideal gas law, the Arrhenius equation, and the Nernst equation.

**Heat capacity** or **thermal capacity** is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).

The **newton-metre** is a unit of torque in the SI system. One newton-metre is equal to the torque resulting from a force of one newton applied perpendicularly to the end of a moment arm that is one metre long. The nonstandard notation *Nm* occurs in some fields.

The **therm** is a non-SI unit of heat energy equal to 100000 British thermal units (Btu). It is approximately the energy equivalent of burning 100 cubic feet – often referred to as 1 CCF – of natural gas.

The following outline is provided as an overview of and topical guide to energy:

**TNT equivalent** is a convention for expressing energy, typically used to describe the energy released in an explosion. The *ton of TNT* is a unit of energy defined by that convention to be 4.184 gigajoules, which is the approximate energy released in the detonation of a metric ton of TNT. In other words, for each gram of TNT exploded, 4184 joules of energy is released.

**Metric units** are units based on the metre, gram or second and decimal multiples or sub-multiples of these. The most widely used examples are the units of the International System of Units (SI). By extension they include units of electromagnetism from the CGS and SI units systems, and other units for which use of SI prefixes has become the norm. Other unit systems using metric units include:

The **energy content of biofuel** is a description of the chemical energy contained in a given biofuel, measured per unit mass of that fuel, as specific energy, or per unit of volume of the fuel, as energy density. A biofuel is a fuel, produced from living organisms. Biofuels include bioethanol, an alcohol made by fermentation—often used as a gasoline additive, and biodiesel, which is usually used as a diesel additive. Specific energy is energy per unit mass, which is used to describe the energy content of a fuel, expressed in SI units as joule per kilogram (J/kg) or equivalent units. Energy density is the amount of energy stored in a fuel per unit volume, expressed in SI units as joule per litre (J/L) or equivalent units.

The **watt** is a unit of power or radiant flux. In the International System of Units (SI), it is defined as a derived unit of 1 kg⋅m^{2}⋅s^{−3} or, equivalently, 1 joule per second. It is used to quantify the rate of energy transfer. The watt is named after James Watt, an 18th-century Scottish inventor.

**Photon energy** is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy.

A **coherent system of units** is a system of units, used to measure physical quantities, which are defined in such a way that the equations relating the numerical values expressed in the units of the system have exactly the same form, including numerical factors, as the corresponding equations directly relating the quantities. Equivalently, it is a system in which every quantity has a unique unit, or one that does not use conversion factors.

- ↑ Johnson, R. D., ed. (2002). "CCCBDB What's a cm-1?".
*Computational Chemistry Comparison and Benchmark DataBase*. National Institute of Standards and Technology. doi:10.18434/T47C7Z . Retrieved 2013-11-13. - ↑ Thompson, A.; Taylor, B. N., eds. (2019). "Appendix B8—Factors for Units Listed Alphabetically".
*NIST Guide for the Use of the International System of Units (SI)*.*National Institute of Standards and Technology*(9th ed.).

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