1.3: Measurement of energy

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Units of Measurement

Energy is measured differently in different industries and parts of the world. The SI unit for energy is the Joule (J). Table 1.3.1. provides some units of energy that are used.

Table 1.3.1. Different measurements of energy
Unit	Definition	Used In	Unit Conversions
British thermal unit (BTU)	Amount of energy needed to raise temperature of 1 pound of water by 1 degF, equivalent to energy found in the tip of a match stick	Heating/cooling industries	1 BTU = 1055 J
Small calorie (cal)	Amount of energy needed to raise temperature of 1 gram of water by 1 degC	Science and engineering	1 cal = 0.003969 BTUs
Food calorie or kilocalorie (Cal, kcal, Calorie)	Amount of energy needed to raise temperature of 1 kilogram of water by 1 degC, often for measuring energy content of food	Nutrition	1 Cal = 1000 cal, 4.187 J, 3.969 BTUs
Joule (J)	Amount of energy smaller than a calorie and BTU, SI unit of energy	Science and engineering	1 J = 0.2388 cal, 0.0009481 BTUs
Kilowatt hour (kWh)	Amount of energy from the steady production or consumption of 1 kilowatt of power over 1 hour	Electrical fields	1 kWh = 3.413 BTUs, 3,600,000 J
Therm	Amount of energy in natural gas	Home heating appliances	1 therm = 100,000 BTUs

Additional Information

When writing BTUs, one uses a base of “10” raised to a particular exponent.

For example:

10,000 BTUs = 10⁴ BTUs
100,000 BTUs = 10⁵ BTUs
1,000,000 BTUs = 10⁶BTUs

More specific notation involves the following:

10,000 BTUs = 10⁴ BTUs
100,000 BTUs = 10⁵BTUs
1,000,000 BTUs =10⁶ BTUs

To express measurements greater than those with a base of 10, you would do the following:

50,000 BTUs = 5 * 10⁴ BTUs
700,000 BTUs = 7 * 10⁵ BTUs
9,000,000 BTUs = 9 * 10⁶ BTUs

Energy Scale

The video below shows the energy used by various countries or processes on a logarithmic scale measured in BTUs.

It is interesting to note that the earth received 24,330 times more energy than the entire world used in 2001. Then one might ask, “Why should anybody worry about energy shortage?" Whether energy is there or not is actually a secondary concern. The real issue is the form in which the energy is available and whether it can be easily converted to the form that we need.