Definition · Plain-language
Units of energy
The SI unit of energy is the joule; other common units — the calorie, kilowatt-hour and electronvolt — each suit a particular field but all measure the same quantity.
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The joule, the SI unit
The SI unit of energy is the joule, symbol J. It is a derived unit, defined as the energy transferred when a force of one newton acts through a distance of one metre — so one joule is one newton metre. Written in base units, that is a kilogram metre squared per second squared. The same unit covers every form of energy, because energy is interchangeable: the joule measures the kinetic energy of a moving object, the potential energy of a raised weight, the heat in a warm body and the electrical energy in a circuit alike. This universality is why a single unit suffices in principle.
Calories, kilowatt-hours and electronvolts
In practice, several other units persist because they are convenient in their fields. The calorie, once defined as the heat needed to warm one gram of water by one degree Celsius, lingers in nutrition and chemistry — the dietary "Calorie" is actually a kilocalorie, about 4,184 joules. The kilowatt-hour, the unit on an electricity bill, is the energy used by a one-kilowatt device in an hour, equal to 3.6 million joules. The electronvolt, tiny by comparison, suits atomic and particle physics. Each is just a different-sized parcel of the same quantity.
Energy, work and power
Energy is closely tied to two related ideas that are sometimes confused with it. Work is energy transferred when a force moves something, and it is measured in the same unit, the joule. Power is the rate at which energy is used or transferred — energy per unit time — and its SI unit is the watt, equal to one joule per second. The kilowatt-hour neatly links the two: a power in kilowatts multiplied by a time in hours gives an energy. Keeping energy (joules) and power (watts) distinct is essential to using the units correctly.
Key facts
At a glance
- SI unit: the joule (J)
- Joule defined: one newton acting over one metre (1 N·m)
- In base units: kilogram metre squared per second squared
- Dietary Calorie: a kilocalorie ≈ 4,184 joules
- Kilowatt-hour: energy of 1 kW for 1 hour = 3.6 million joules
- Power differs: the watt is energy per second, not energy itself
Common misconceptions
What people often get wrong
Often heard: The watt is a unit of energy.
Actually: The watt is a unit of power — the rate of using energy, equal to one joule per second. Energy itself is measured in joules; a watt tells you how fast energy is used.
Often heard: The dietary Calorie is the same as the scientific calorie.
Actually: The food "Calorie" with a capital C is a kilocalorie — a thousand small calories, about 4,184 joules. The two differ by a factor of a thousand, a frequent source of confusion.
Often heard: A kilowatt-hour is a unit of power.
Actually: It is a unit of energy. It equals a power of one kilowatt sustained for one hour — 3.6 million joules. Power multiplied by time gives energy, not the other way around.
Going deeper
