Units of Energy

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UNITS OF ENERGY

The joule, symbol J, is the unit of energy in the science community. It is not widely used outside the science community in the United States, but it is elsewhere. For example, in the United States, the energy content of a food product is likely expressed in food calories. This energy could be expressed in joules, and that is being done in many parts of the world. A Diet Coca Cola in Australia is labeled Low Joule rather than Low Cal as in the United States. To obtain a feeling for the size of a joule, consider the following statistics:

the sound energy that is in a whisper is about 0.01 joules,
the kinetic energy of a 1,000-kilogram car traveling 25 meters per second (55 miles per hour) is about 300,000 joules,
the energy from burning a barrel of oil is 6,000,000 joules,
the annual energy use in the United States is about 100 billion billion joules, and
the daily energy input to the Earth from the sun is about 10,000 billion billion joules.

Even if the use of units of joules were universal, the numbers involved in energy discussions would be large and cumbersome. Therefore, it is customary to use powers of ten notation and prefixes for powers of

Numerical Unit	Power of Ten	Prefix	Symbol
million trillion	10¹⁸	exa	E
trillion	10¹²	tera	T
billion	10⁹	giga	G
million	10⁶	mega	M
thousand	10³	kilo	k
hundredth	10^-2	centi	c
thousandth	10^-3	milli	m
millionth	10^-6	micro	μ

ten. For example, the energy from burning a barrel of oil is 6 × 10⁶ joules, which can be expressed as 6 megajoules or, symbolically, 6 MJ. Table 1 summarizes the powers of ten, prefixes, and symbols usually encountered in energy considerations.

The gasoline tank on an automobile holds about 15 gallons, and the automobile can travel about 300 miles before the tank is empty. Even though the gasoline was purchased for its energy content, the driver probably did not know that the energy content of 15 gallons of gasoline is 2 × 10⁹ J = 2 GJ. For reasons like this, there exists a variety of energy units and energy equivalents. The unit and equivalent depends on the type of energy commodity in question. A homeowner pays an electric utility for electric energy, and the

Special Unit	Study area of main use	Symbol	Equivalent in joules	Other units
Kilowatt hour	electricity	kWh	3,600,000	3413 Btu
Calorie	heat	cal	4.186
Kilocalorie (food calorie)	heat	kcal	4,186	1000 cal
British Thermal unit	heat	Btu	1,055	252 cal
Electron volt	atoms, molecules	eV	1.60x10^-19
Kilo-electron Volt	X-rays	keV	1.60x10^-16	1000 eV
Mega-electronVolt	nuclei, nuclear radiations	MeV	1.60x10^-13	1000 keV
Quadrillion	energy reserves	quad	1.055x10²¹	10¹⁵Btu
Quintillion	energy reserves	Q	1.055x10²¹	10¹⁸Btu
Energy equivalents 1 gallon of gasoline = 126,000 Btu 1 cubic foot of natural gas = 1030 Btu 1 pound of bituminous coal = 13,100 Btu 1 42-gallon barrel of oil = 5,800,000 Btu 1 therm = 100,000 Btu Variations of these energy equivalents will appear in the literature. The values listed here are typical.

unit is likely a kilowatt-hour (kWh). A politician interested in imported oil will likely talk in terms of barrels of oil. Table 2 summarizes some special energy units and their equivalents.

Joseph Priest