Saturday, October 21, 2006

Energy units - counting numbers of zeroes

Energy comes in many different forms - kinetic, thermal, chemical, magnetic, electric. From the humble electron-volt (eV) used to describe nuclear processes, to the tera-watt-hour (TWh) output from nuclear power stations, we count 34 orders of magnitude. Oil experts like to think of energy in terms of tonnes of oil equivalent (toe), coal experts in tonnes of coal equivalent (tce) and power engineers swear by the kilo-Watt-hour (kWh) and its multiples MWh - GWh - TWh. All these units are measures of energy, and can be expressed in the SI unit of energy, the Joule. Many of the units on this page, while commonly used, are not part of the International System of Units (SI).

Hereby an overview of some of the energy units that are frequently used (source):


Unit frequently used in physics. 1 erg = 10E-7 Joule.

eV (electronVolt)

1 eV = 1.6 * 10E-19 Joule. It is the energy that an electron would require when dropping through a potential of 1 volt.


A barrel of oil is 42 gallons, or 159 liters. The chemical energy obtained when burning a barrel of oil is 6,120 megajoule (MJ), or 1,700 kWh. Hence, there is about 10 Kwh (thermal) in a liter of oil.

toe (tonne of oil equivalent)

A tonne of oil equivalent is roughly 7 barrels. Using IEA's unit convertor (see below), one can verify it to be equal to 41,868 MJ or 11,630 kWh (thermal).

tce (tonne of coal equivalent)

Using IEA's unit convertor (see below), a tonne of coal equivalent is 0.7 toe. Hence, a tce = 29,308 MJ or 8,141 kWh. There are about 8 kWh in a kg of coal.

BTU (British Thermal Unit

A unit defined as that amount of energy that will warm one pound of water by one degree Fahrenheit. 1 BTU = 1055 Joule.


The amount of energy required to warm 1 g of water by 1 degree Celsius. 1 cal = 4.186 J. The human body needs about 2000 Kcal per day, i.e. about 3 GJ or 850 kWh of food energy per year.


A quadrillion BTU is a Quad (10E15). Hence, it's 1,055 quadrillion Joule, or 293 billion kWh. This is the annual primary energy input for 12 thermal power stations with 33% conversion efficiency.

kWh (kiloWatt-hour)

A kWh is the amount of energy consumed by a 1 kW device during a period of 1 hour. 1 kWh = 3,600,000 J.


The metric unit for energy. It is the amount of energy consumed in a 1 Watt device (e.g. a torch bulb) during 1 sec. 1J = 1 Ws = 1 Nm (Newton meter). The Newton is the basic metric unit for force, equivalent to the weight of a mass of approximately 102 grams. A Nm is the mechanical energy required to exert a force of 1 N over a distance of 1 m.

unit convertors

There are many more units for expressing energy. See following links:

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