Energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

is defined via work, so the SI unit of energy is the same as the unit of work – the

joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...

(J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, is equal to 1

newton metre Newton most commonly refers to: * Isaac Newton (1642–1726/1727), English scientist * Newton (unit), SI unit of force named after Isaac Newton Newton may also refer to: People * Newton (surname), including a list of people with the surname * N ...

and, in terms of

SI base unit The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which al ...

s :

1\ \mathrm = 1\ \mathrm \left( \frac \right ) ^ 2 = 1\ \frac

An energy unit that is used in

atomic physics Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned wit ...

particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, and

high energy physics Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the stu ...

is the

electronvolt In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an Voltage, electric potential difference of one volt in vacuum ...

(eV). One eV is equivalent to . In

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

, the unit cm⁻¹ ≈ is used to represent energy since energy is inversely proportional to wavelength from the equation

E = h \nu = h c/\lambda

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

British imperial / US customary units

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 Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...

-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ). Log10 ratios between energy units

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.

Electricity

A unit of electrical energy, particularly for utility bills, is the

kilowatt-hour A kilowatt-hour ( unit symbol: kW⋅h or kW h; commonly written as kWh) is a non-SI unit of energy equal to 3.6 megajoules (MJ) in SI units, which is the energy delivered by one kilowatt of power for one hour. Kilowatt-hours are a comm ...

(kWh); one kilowatt-hour is equivalent to . Electricity usage is often given in units of kilowatt-hours per year or other periods. This is a measurement of average power consumption, meaning the average rate at which energy is transferred. One kilowatt-hour per year is around 0.11 watts.

Natural gas

Natural gas is often sold in units of energy content or by volume. Common units for selling by energy content are

s or therms. One therm is equal to about . 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.

Food industry

The

calorie The calorie is a unit of energy that originated from the caloric theory of heat. The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter o ...

is defined as the amount of

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

necessary to raise the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of one

gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram. Originally defined in 1795 as "the absolute Mass versus weight, weight of a volume ...

of water by 1

Celsius The degree Celsius is the unit of temperature on the Celsius temperature scale "Celsius temperature scale, also called centigrade temperature scale, scale based on 0 ° for the melting point of water and 100 ° for the boiling point ...

degree, from a temperature of , at a

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

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 Food energy is chemical energy that animals and humans derive from food to sustain their metabolism and muscular activity. Most animals derive most of their energy from aerobic respiration, namely combining the carbohydrates, fats, and protein ...

is measured in ''large calorie''s (a ''large calory'' is a kilocalory, equal to ), sometimes written capitalized as . In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information.

Atom physics and chemistry

In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, units

s (eV). One electronvolt (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 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 the field of computational chemistry since such units arise directly from the calculation algorithms without any need for conversion. Historically Rydberg units have been used.

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\sim 2\cdot 10^\ \mathrm\ \mathrm

being the proportionality constant.

Explosions

A gram of TNT releases upon explosion. To define the tonne of TNT, this was standardized to giving a value of for the tonne of TNT.

References

{{Energy footer Conversion of units of measurement