**James Prescott Joule** (1818 − 1889) was a self-educated British physicist
and brewer whose work in the midnineteenth 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 m ^{2}s^{−2}.**

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=10^{3}), mega (M=10^{6}), giga (G=10^{9}), tera (T=10^{12}),
peta (P=10^{15}),

Exa (E=10^{18}).

Thus, a kiloJoule (kJ) is 1000 Joules and a megaJoule (MJ) 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 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.

1 cal_{IT} = 4.1868 J, International Table calorie

1 cal_{th} = 4.184 J, thermochemical calorie

1 cal_{15} ≈ 4.1855 J, calorie to heat 1 cm^{3} water 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 E_{h} ≈ 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 = 29.3076×10^{9} J, ton of coal equivalent, 7000 kcal_{IT}

1 toe = 41.868×10^{9} J, ton of oil equivalent, 10000 kcal_{IT}

Calories and/or kilocalories [cal and/or kcal] were historically often used to measure heat (energy) and are still used fot thi 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.

Symbol | Exponential | Prefix | Quantity |
---|---|---|---|

k | 10^{3} |
kilo | thousand |

M | 10^{6} |
mega | million |

G | 10^{9} |
giga | billion |

T | 10^{12} |
tera | trillion |

P | 10^{15} |
peta | quadrillion |

E | 10^{18} |
exa | quintillion |

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

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 about 31.56 kWh/d,
- the annual consumption of 1 GJ/yr corresponds to the daily consumption of about 0.7605 kWh/d.

The unit conversions indicated on this page can be performed with a calculator. One can also find conversion calculators on the internet such as the International Energy Agency, unitconversion.org and at Robert Fogt's page.