Joule [J]: the basic unit of energy in the SI system. It corresponds to the work done by a force of one newton moving an object 1 meter in the direction of the force.

Megajoule [MJ]: one million joules. 1 MJ is 1000 kJ or 1,000,000 J.

Calorie [cal]: a unit of thermal energy equal to the amount of heat needed to raise the temperature of 1 g of water by 1 °C at atmospheric pressure. One calorie is about 4.184 J.

Kilocalorie [kcal]: one thousand calories, or 1000 cal. A unit often used to express the energy value of food or the body's energy requirements. One kilocalorie is about 4.184 kJ.

Watt-hour [Wh]: a unit of electrical energy equal to the amount of energy used by a device with a power of 1 W operating for one hour. One watt-hour is 3600 J.

Kilowatt-hour [kWh]: one thousand watt-hours, or 1000 Wh. A unit often used to express household or business electricity consumption. One kilowatt-hour is 3.6 MJ.

Megawatt-hour [MWh]: one million watt-hours, or 1,000,000 Wh. A unit often used to express large-scale electricity production or delivery. One megawatt-hour is 3.6 GJ.

British thermal unit (BTU): a unit of thermal energy equal to the amount of heat needed to raise the temperature of a pound (about 454 g) of water by one degree Fahrenheit. One BTU is about 1055 J.

Therm: a unit of thermal energy equal to one hundred thousand British thermal units (BTU), or about 105,506,000 J. A unit often used to express natural gas consumption.

Tonne of oil equivalent [toe]: a unit of primary energy equal to the amount of energy obtained from burning one tonne of crude oil with an average calorific value of about 41,868,000 kJ (10 MWh). A unit often used to compare different energy sources and forms.