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Units of Energy

Because energy is defined via work, the SI unit for 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, 1 joule is equal to 1-newton meter and, in terms of SI base units

James Prescott Joule (1818 − 1889) was a self-educated British physicist and brewer whose work in the mid-nineteenth century contributed to the establishment of the energy concept. The international unit of energy bears his name:

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It takes about 1 J to raise a 100-g-apple 1 m. Energy units can be preceded by various factors, including the following:

kilo (k=103), Mega (M=106), Giga (G=109), Tera (T=1012), Peta (P=1015), Exa (E=1018).

Thus, a kiloJoule (kJ) is 1000 Joules and a MegaJoule (MJ) is 1,000,000 Joules.

The words “power” and “energy” are often used interchangeably. While energy in the ability to do work, power is the rate at which energy is transferred, used, or transformed.

A related unit is the Watt, which is a unit of power (energy per unit time). Power units can be converted to energy units through multiplication by seconds [s], hours, [h], or years [yr].

For example, 1 kWh [kilowatt hour] = 3.6 MJ [MegaJoule]. With 1 kWh, about 10 liters of water can be heated from 20 ºC to the boiling point.

There are many other energy units besides the “Système International d’Unités (SI)”.

  • A “ton of coal equivalent” (tce) is frequently used in the energy business. 1 tce equals 8.141 MWh. It means that the combustion of 1 kg of coal produces the same amount of heat as electrical heating for one hour at a rate of 8.141 kW.
  • 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 = 0.000123986 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.
  • When discussing amounts of energy released in explosions or bolide impact events, the TNT equivalent unit is often used.

More Units of Energy

1 calIT = 4.1868 J, International Table calorie
1 calth = 4.184 J, thermochemical calorie
1 cal15 ≈ 4.1855 J, calories to heat from 14.5 °C to 15.5 °C
1 erg = 10−7 J, cgs [centimeter-gram-second] unit
1 eV ≈ 1.60218 × 10−19 J, electron volt
1 Eh ≈ 4.35975 × 10−18 J, Hartree, atomic energy unit
1 Btu = 1055.06 J, British thermal unit according to ISO, to heat 1 pound water from 63 °F to 64 °F
1 tce = 29.3076 × 109 J, ton of coal equivalent, 7000 kcalIT
1 toe = 41.868 × 109 J, ton of oil equivalent, 10000 kcalIT

Calories and/or kilocalories [cal and/or kcal] were historically often used to measure heat (energy) and are still used for this sometimes today. Heating a gram of water 1º C requires 1 cal. Different definitions are often the result of inconsistent starting temperatures of the heating.

Multiplication Table of Units

Symbol Exponential Prefix Quantity
k 103 kilo thousand
M 106 Mega million
G 109 Giga billion
T 1012 Tera trillion
P 1015 Pet quadrillion
E 1018 Exa quintillion

The unit Megagram is not used since there is a special name for one million grams, one ton(t): 1 t = 1000kg.

Multiplication of the Units of Power with Units of Time

When the Watt is multiplied by a unit of time, an energy unit is formed as follows: 1 Ws = 1 J.
The use of the kilowatt-hour is more common: 1 kWh = 3600 kWs = 3.6 MJ.
Besides the second [s] and the hour [h], the day [d] and the year [yr] are also used, with 1 yr = 365.2425 d = 31,556,952 s.

So, for example, energy of one Megawatt-year can be written as 1 MWyr = 31.557952 TJ (TeraJoule).
The annual consumption of 1 toe/yr corresponds to the daily consumption of 31.557952 kWh/d.


Here is a summary of all the units of energy and related terms, you need to know!!

Barrel of oil

Barrel is unit of measurement of volume. Generally, it is used to describe the production or consumption of crude oil by any entity. 1 barrel of oil (1 bbl) = 42 US gallon = 159 litres.


The approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius. This unit is used for measuring heat energy. One calorie is equal to 4.184 joules. It is often used instead of joules when dealing with the energy released from food.


A unit for measuring the rate of work (or power) equivalent to 33,000 foot-pounds per minute or 746 watts. The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per second is approximately equivalent to 745.7 watts.

Joule (J)

A standard International System of Units of energy; 1055 Joules is equal to 1 BTU.

Kilowatt-hour (kWh)

A unit of energy equal to one kW applied for one hour; running a one kW equipment for one hour would dissipate one kWh of electrical energy as heat. The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time. 1 kWh = One thousand watt hours.

Kilowatt (kW)

One thousand watts of electricity.

Megajoule (MJ)

One million Joules.

Megawatt (MW)

One Million Watts; a modern coal plant will have a capacity of about 1,000 MW.

Unit of electricity

1 unit of electricity = 1 kWh. The electricity used is generally charged for in “unit of electricity’. Kilowatt-hours is the product of watts x time (one kilowatt = 1000 watts). A two-kilowatt heater switched on for three hours will have used six kilowatt-hours of electricity.

Volt (V)

The volt is the International System of Units (SI) measure of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance.

Watt (Electric)

The electrical unit of power. The rate of energy transfer equivalent to 1 ampere of electric current flowing under a pressure of 1 volt at unity power factor.

Watt (Thermal)

A unit of power in the metric system, expressed in terms of energy per second, equal to the work done at a rate of 1 joule per second.

Watthour (Wh)

The electrical energy unit of measure equal to 1 watt of power supplied to, or taken from, an electric circuit steadily for 1 hour.

Hope, you have grasped the concept in a vivid manner.

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