Energy is defined as the ability to do work. It is measured primarily in Joules (J), British Thermal Units (BTU)  and calories (cal).

The Système International d'Unités (SI) unit for energy is the joule (J), after James Joule, who demonstrated that work can be converted into heat. Lifting a medium-sized potato a distance of 1 m (3.28 ft) would require approximately one joule of energy. Energy is often expressed as the calorie (cal), which is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius. One calorie is equal to 4.184 joules. The Calorie (Cal), which is used to express the energy in food, is 1,000 calories.

1 cal =4.184J

1 kJ = 1,000 J

1kcal= 4.184 kJ

1kcal = 1,000 cal = 1 Calorie (food)

The Joule is named for the British physicist James Prescott Joule (1818-1889), who demonstrated the equivalence of mechanical and thermal energy in a famous experiment in 1843. Although Joule pronounced his name "jowl", the unit is usually pronounced "jool" or "jew'l".

Examples