The official unit of the International System (SI) is the Joule (1 J), equal to the work done by a force of 1 Newton whose point of application moves 1 m in the direction of the force.

However, this value is not very meaningful, and is therefore little used in practice.

Historically, the calorie has long been used to measure heat quantities. It represents the energy amount required to raise the temperature of 1 gram of water by 1 °C at atmospheric pressure. As this value varies slightly depending on the initial temperature of the water, there are differences between the used definitions. We retain the following equivalence:

1 cal = 4.187 J (International Steam Table calorie)

An equivalent definition is used in the British system, still widely used especially in the United States: the British thermal unit or BTU represents the amount of heat required to raise by one Fahrenheit degree the temperature of 1 lb (pound) of water (0.4535924 kg).

With the same convention as for calorie, we have the following equivalence:

1 BTU = 1055.6 J

This implies:

1 BTU = 251.984 cal

The unit of power corresponding to the Joule energy unit is the Watt: a power of 1 W used for 1 second releases energy amounting to 1 Joule.

1 kilowatt-hour (kWh) is the energy amount released during one hour by a 1 kW power device.

Kilowatt and kilowatt-hours are units of power and energy most often used to measure electricity.

By definition:

1 Wh = 3600 J

1 kWh = 3.6 10⁶ J = 3.6 MJ

To study the consumption and production of energy at the level of a country, the units used are rarely Joule or calories.

Due to the importance of oil in the energy balance of many countries, the amount of energy is frequently expressed as an oil equivalent: tonnes of oil equivalent (toe), kilograms of oil equivalent (koe), or barrels of oil equivalent (boe).

Other units are also used. This depends on the energy sources considered and their characteristics, which often vary a lot depending on the case. For natural gas, one uses cubic meters (m³) or cubic feet (cf) under standard conditions of temperature and pressure (25 ° C, 1 atm for m³, 60 ° F, 30 inHg for cf). The equivalence between these two units is:

• 1 Nm³ = 37.24 Ncf

• 1 Nm³ = 38 MJ

• 1 cf = 1,020 Btu

Primary or final energy is often reported in terms of millions or billions of tonnes of oil equivalent, or, in countries using the British system, in quadrillion Btu (quad) :

1 quad = 1 10¹⁵ Btu = 1.055 10¹⁸ J

Conversions between these units are possible, but they are necessarily arbitrary, so that values may slightly differ depending on the authors. The IEA / OECD choose the following equivalences:

• 1 tep = 41.868 GJ = 39.68 MBtu = 11.63 MWh

• 1 bep = 5.8 MBtu = 0.146 tep

• 1 tep = 6.84 bep

• 1 quad = 25.2 Mtep

Note that quite different toe / boe equivalence values may be found in the literature, including 1 toe = 7.33 boe, representing a difference of 7.14%, which can be explained by variations in the density and calorific value of the crudes used as references. For instance, BP uses a conversion factor of about 7 in the BP statistical review of world energy.

As tonnes are most frequently used, let us point out that one must distinguish them from tons, using the following conversions :

1 short ton (ton) = 2,000 lb

1 metric ton (tonne) = 1,000 kg

1 ton = 0.907185 tonne

Equivalences below allow one to convert figures between the most frequently encountered units.

OIL:

• 1 tonne = 6.84 barrels

• 1 barrel/day for 1 year ≈ 50 t

• 1 toe = 41.868 GJ

• 1 toe = 11,628 kWh

• 1 boe = 5.8 MBtu

NATURAL GAS:

• 1 m³= 38 MJ

• 10⁶ m³≈ 910 toe

• 1 cf = 1020 Btu

• 10⁹ cf ≈ 25 toe

COAL:

• 1 t = 0,7 toe (coal)

• 1 t = 0,55 toe (sub-bituminous coal)

• 1 t = 0,25 toe (lignite)

ELECTRICITY:

• 1 MWh = 0.2606 toe (equivalent to production)

• 1 MWh = 0.086 toe (equivalent to consumption)