Why is 1 kWh 3 6 MJ

The unit of measure of energy

James Prescott Joule (1818 - 1889), British physicist, autodidact and brewery owner, made a significant contribution to the establishment of the concept of energy with his work in the middle of the 19th century. The internationally valid unit of energy bears his name:

1 joule [J] = 1 watt second [Ws] = 1 VAs = 1 Nm = 1 kg m2 s−2.

The power unit watt [W] is named after James Watt and the power unit Newton 4930 after Isaac Newton.

With 1 J you can lift an apple weighing 100 g on the ground to a height of about 1 m. The energy units and the electrical power have watts as factors

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

The power unit watt becomes the energy unit joule by multiplying it by the second [s], hour [h] or year [a], see below. For example, 1 kWh [kilowatt hour] = 3.6 MJ. With 1 kWh you can heat about 10 liters of water from 20 ° C to the boiling point of about 100 ° C.

Other units of measure of energy

The "Système International d'Unités (SI) defines the so-called SI units legally prescribed for use in science and technology in Germany and other countries. In addition to these units, a variety of historically developed units is still in use. The hard coal unit (tSKE) is called English tce = tone of coal equivalent, and 1 tSKE corresponds to exactly 8.141 MWh. Burning 1 kg of hard coal generates the same amount of heat as an electric heater with 8.141 kWh. The crude oil unit (tRÖL) is called toe, ton of oil equivalent, and 1 tRÖL corresponds to exactly 11.63 MWh.

The following conversions apply:
1 calIT = 4.1868 J, international calorie, International table calorie
1 calth = 4.184 J, thermochemical calorie, thermochemical calorie
1 cal15 ≈ 4.1855 J, calorie for heating from 14.5 ° C to 15.5 ° C
1 erg = 10−7 J, cgs unit
1 eV ≈ 1.60218 × 10−19 J, electron volts
1 E.H ≈ 4,35975 × 10−18 J, Hartree, atomic unit
1 Btu = 1055.06 J, British thermal unit according to ISO, to heat 1 pound water from 63 ° F to 64 ° F
1 tSKE = 1 tce = 29.3076 × 109 J, ton coal unit, tce, tone of coal equivalent, 7000 kcalIT
1 tRÖL = 1 toe = 41.868 × 109 J, ton unit of crude oil, toe, ton of oil equivalent, 10000 kcalIT

In 1850 it was determined that the amount of heat of one kilocalorie [1 kcal] warms one kilogram of water by 1 ° C. Calorie or kilocalorie [cal or kcal] were then legal units until 1948. They are still in use today (mostly with the conversion given above for the international calorie) alongside the joule as a unit of measurement for the amount of heat.

Multiplication of units of measure

symbolexponent number prefixGermanAmer. EnglishBrit. English
G9109Gigabilliontrillionthousand million
P.151015PetaBilliardsquadrillionthousand billion

The ton (1 t = 1000 kg = 1 Mg) is not an SI unit, but is permitted as a legal unit. The SI unit Mg (megagram) is rarely used for this mass. The masses dt (deciton, formerly quintals dz), kt (kiloton) and Mt (megaton) are also associated with the ton.

Multiplication of performance units with time units

The power unit watt (with the above-mentioned multiples) results in a unit of energy when multiplied by a unit of time, e.g. B. 1 Ws = 1 J. The kilowatt hour is very common: 1 kWh = 3600 kWs = 3.6 MJ. In addition to the second [s] and the hour [h], the Gregorian year [a] with 1 a = 365.2425 d = 31 556 952 s is used as the time unit.

This results, for example, in the unit megawatt year with 1 MWa = 31.557952 TJ [terajoules].
The annual consumption of 1tRÖL (1 toe / a) results in a daily consumption of around 31.84 kWh / d.
The consumption of 1 GJ / a results in about 0.7605 kWh / d.

Conversion of energy units

The above information enables the conversion of energy units with the pocket calculator.
If you don't want to go through this trouble, you can use numerous "converters" on the Internet.
There are offers e.g. B. from International Energy Agency, Robert Fogt and unitconversion.org.


Last change: 01.09.2020