Other Energy Units

The joule, while standard among scientists, is not the only energy unit in existence! Another unit is the erg, equivalent to one ten-millionth (0.0000001) of a joule. The erg is used in lab experiments involving small amounts of expended energy.

Most folks have heard of the British thermal unit (Btu), equivalent to 1055 joules. This is the energy unit commonly used to define the cooling or heating capacity of air-conditioning equipment. To cool your room from 85 to 78°F needs a certain amount of energy, perhaps best specified in Btu. If you are getting an air conditioner or furnace installed in your home, an expert will come look at your situation, and determine the size of air-conditioning/heating unit that best suits your needs. That person will likely tell you how powerful the unit should be in terms of its ability to heat or cool in Btu per hour (Btu/h).

Physicists also use, in addition to the joule, a unit of energy called the electron volt (eV). This is a tiny unit of energy, equal to just 0.00000000000000000016 joule (there are 18 zeroes after the decimal point and before the 1). The physicists write 1.6 × 10−19 to represent this. It is the energy gained by a single electron in an electric field of 1 V. Machines called particle accelerators (or atom smashers) are rated by millions of electron volts (MeV), billions of electron volts (GeV), or trillions of electron volts (TeV) of energy capacity.

Another energy unit, employed to denote work, is the foot-pound (ft-lb). This is the work needed to raise a weight of one pound by a distance of one foot, not including any friction. It’s equal to 1.356 joules.

Conversion factors between joules and various other energy units.
Unit To convert energy in this unit to energy in joules, multiply by To convert energy in joules to energy in this unit, multiply by
British thermal units (Btu) 1055 0.000948
Electron volts (eV) 1.6 × 1019 6.2 × 1018
Ergs 0.0000001 (or 10−7) 10,000,000 (or 107)
Foot-pounds (ft-lb) 1.356 0.738
Watt-hours (Wh) 3600 0.000278
Kilowatt-hours (kWh) 3,600,000 (or 3.6 × 106) 0.000000278 (or 2.78 × 10−7)

All of these units, and conversion factors, are given in above Table. Kilowatt-hours and watt-hours are also included in this table. In electricity and electronics, you need to be concerned only with the watt-hour and the kilowatt-hour for most purposes.