Heat energy can be measured in joules (J), calories (cal), or kilocalories (kcal). The joule is the unit used to measure heat in the International System of units. One joule equals the work done by a force of one newton acting through a distance of one meter. One calorie is the amount of heat that is needed to raise the temperature of 1 g of pure water 1° C. A joule is about 1/4 of a calorie. See the equations at bottom of page for exact comparisons.

The term Calorie (with a capital c) used in nutrition is equal to one kilocalorie (1,000 calories). So the statement that 10 g of sugar has 41 Calories means that burning 10 g of sugar will produce 41 kcal (41,000 calories) of heat energy.

1J = 0.239 cal and 1 cal = 4.18J

The British thermal unit (Btu) is a larger unit than the kilocalorie. One Btu is the amount of energy required to raise the temperature of one pound of pure water 1° F. The Btu is often used in measurements of the energy produced in internal combustion machines. Electric power is measured in watts. When talking about relatively large amounts of power, kilowatts (1,000 watts) are used. Kilowatt-hours, a measure of electrical energy, can be related directly to Btu (1 kWh = 3,413 Btu), allowing comparisons between electrically powered systems and combustion heat systems.

Click here to see Figure 5 Heat, fuel and electricity units.