Energy Units Explained, From Joules to kWh

The joule (J) is the SI unit of energy. One joule is the work done by a force of one newton acting through one meter: 1 J = 1 N·m. It is also the energy of one watt for one second.

A joule is small, so everyday values are often given in kilojoules (1 kJ = 1000 J) or megajoules (1 MJ = 1 000 000 J). Every other unit on this page is defined as an exact number of joules.

The calorie (cal) was first tied to the heat needed to warm one gram of water by one degree Celsius. Because that amount depends on the starting temperature, the calorie is now defined directly in joules.

This site uses the thermochemical calorie: 1 cal = 4.184 J exactly, and 1 kcal = 1000 cal = 4184 J exactly. Another definition, the International Table calorie, is 4.1868 J. The two differ by less than 0.1%.

The "Calorie" with a capital C on food labels is a kilocalorie. A food with 250 Calories contains 250 kcal, which is 1046 kJ. Many countries print both kJ and kcal on labels.

A watt-hour (Wh) is the energy of one watt used for one hour: exactly 3600 J. A kilowatt-hour (kWh) is 1000 Wh, exactly 3.6 MJ.

Electricity bills are charged in kilowatt-hours. A 2000 W heater running for 3 hours uses 6 kWh, which is 21.6 MJ. Battery capacities in phones, laptops and electric cars are also given in Wh or kWh.

The British thermal unit (BTU) is a US customary unit for heat. It was first defined as the heat needed to warm one pound of water by one degree Fahrenheit. This site uses the International Table BTU, exactly 1055.05585262 J, or about 1.05506 kJ.

BTU figures appear on furnaces, boilers, water heaters and gas grills in the United States. Air conditioner and heater ratings in "BTU" are usually BTU per hour, a unit of power, not energy. One kilowatt-hour is about 3412.14 BTU.

The electronvolt (eV) is the energy an electron gains when it moves through a potential difference of one volt. It is exactly 1.602176634 × 10⁻¹⁹ J. It is used in atomic, nuclear and particle physics, where energies are tiny, and with prefixes such as keV, MeV and GeV.

• Reading a food "Calorie" as a small calorie. It is a kilocalorie, 1000 times larger.
• Mixing up kilowatts and kilowatt-hours. The kilowatt is power, a rate. The kilowatt-hour is energy, an amount.
• Treating an air conditioner rating in BTU as energy. It is almost always BTU per hour.
• Assuming every calorie is the same. Check whether a source uses the thermochemical or the International Table calorie when precision matters.

• 500 kJ is 119.503 kcal: divide by 4.184.
• 100 000 BTU is 29.3071 kWh.
• A 50 Wh battery stores 180 kJ.