Energy units explained
Every unit here measures the same thing: the capacity to do work or move heat. They differ only in which trade defined them. The one real trap is the word calorie, which names two quantities a thousand apart.
Joule J
Base unit — every factor on this page is measured against it
The SI unit: the work done pushing with one newton over one metre. It is small — lifting an apple a metre takes about one joule — which is why everything practical is quoted in thousands or millions of them.
Kilojoule kJ
1 kJ = 1,000 J Exact
A thousand joules. European food labels lead with kilojoules, so a chocolate bar is around 1,000 kJ. It is also the unit of choice for chemistry and for anything where the joule would need too many zeros.
Megajoule MJ
1 MJ = 1,000,000 J Exact
A million joules. The scale of a household appliance running for an hour, or the energy in a small amount of fuel. A litre of petrol holds roughly 34 MJ.
Watt-hour Wh
1 Wh = 3,600 J Exact
One watt drawn for one hour, or 3,600 joules. The unit for batteries: a phone battery is around 15 Wh, a laptop 60, which is why airlines quote a watt-hour limit on what you can carry.
Kilowatt-hour kWh
1 kWh = 3,600,000 J Exact
A thousand watt-hours, and the unit your electricity bill is priced in. One kilowatt for one hour — a 1,000 W heater for sixty minutes, or a 100 W bulb for ten hours. Exactly 3.6 million joules.
Calorie cal
1 cal = 4.184 J Exact
The thermochemical calorie: exactly 4.184 joules, roughly the heat needed to warm one gram of water by one degree Celsius. This is the physics calorie, and it is a thousand times smaller than the one on a food label.
Kilocalorie kcal
1 kcal = 4,184 J Exact
A thousand calories, and what a nutrition label means by Calorie with a capital C. A 250 Calorie chocolate bar holds 250 kcal, or about 1,046 kJ. The capital letter is the only thing distinguishing them in writing, and it is very often dropped.
British thermal unit BTU
1 BTU = 1,055.06 J Exact
The British thermal unit: roughly the heat needed to raise one pound of water by one degree Fahrenheit, about 1,055 joules, or the energy in a kitchen match. Air conditioners and boilers are rated in BTU per hour.
Therm thm
1 thm = 105,505,585.26 J Exact
A hundred thousand BTU. The unit gas is billed in across the US and, historically, the UK. One therm is roughly the gas a modern boiler burns in a couple of hours of continuous running.
Foot-pound ft·lbf
1 ft·lbf = 1.355818 J Exact
One pound of force applied over one foot. Mostly seen in ballistics as muzzle energy. Do not confuse it with the pound-foot of torque, which is dimensionally different despite looking almost identical written down.
Electronvolt eV
1 eV = 1.6022e-19 J Exact
The energy one electron picks up crossing a one-volt potential difference — about 1.6 times ten to the minus nineteen joules. Sized for single particles, which is exactly why particle physics uses it instead of carrying twenty leading zeros around.