Units of Energy

James Prescott Joule portrait

James Prescott Joule, pictured right, was a self-educated British physicist and brewer whose work in the middle of the 19th century contributed to the establishment of the energy concept. The international unit of energy bears his name:

1 Joule [J] = 1 Watt-second [Ws] = 1 V A s = 1 N m = 1 kg m2 s−2

It takes 1 J to raise a 102-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 is 1,000,000 Joules.

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 with 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.

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 commonly used to measure heat (energy) in the past and are still used sometimes today. Heating a gram of water 1 ºC 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 Mg (Megagram) is not used, since there is a special name for one million grams (equivalently 1000 kg), one ton (t): 1 t = 1000 kg.

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 year is also used, with 1 yr = 365.2425 d = 31,556,952 s. So, for example, energy can be written as 1 MWyr = 31.557952 TJ.

Conversion of Energy Units

The unit conversions indicated on this page can be performed with a calculator. One can also find conversion calculators in the internet like from International Energy Agency, unitconversion.org and Robert Fogt.