What is kilowatt hour?

A kilowatt hour is a unit of energy equal to 1,000 watt hours. It is the standard billing unit used by electricity providers worldwide to measure household and commercial energy consumption.

Real-world uses

The kilowatt-hour is the standard unit for electricity billing worldwide. A typical household uses 250–1,000 kWh per month. EV charging, home appliance energy consumption, and solar panel production are all expressed in kWh. Utility bills list total kWh consumed and price per kWh.

History

The kilowatt-hour became the standard billing unit for electricity in the early 20th century as public electricity grids were established. Its use is mandated for utility metering in most countries. It equals exactly 3,600,000 joules (3.6 MJ).

Common mistakes

Writing "kW/h" instead of "kWh"—the slash implies division (kW per hour), which is a rate of power change, not an energy unit. Also confusing kWh with kW: a 2 kW heater running for 4 hours uses 8 kWh of energy, not 8 kW.

What is kilojoule?

A kilojoule is a unit of energy equal to 1,000 joules. It is commonly used in nutrition to express the energy content of food, as well as in thermodynamics and mechanical engineering.

Real-world uses

Kilojoules are the standard energy unit for food nutrition labelling in Australia, New Zealand, and parts of Europe (alongside kilocalories). A typical adult daily intake is about 8,700 kJ. They are also used in chemistry for reaction enthalpies and combustion energies.

History

The kilojoule became important in nutrition labelling as countries adopted SI-based energy reporting. Australia was among the first to mandate kJ on food labels in the 1970s. The EU requires both kJ and kcal on nutrition labels.

Common mistakes

Confusing kilojoules with kilocalories—1 kcal = 4.184 kJ. Food labels showing "2000 kJ" are about 478 kcal, not 2000 kcal. Also, some people incorrectly assume kJ and kW are the same thing; one is energy, the other is power.

When is this conversion used?

Electricity bills use kilowatt-hours while physics calculations use joules. Converting between them is essential for energy audits, comparing appliance efficiency, and understanding power consumption. Nutritional information is labelled in kilocalories in some countries and kilojoules in others, making this conversion important for dietary planning and food science.

Worked examples

1 kilowatt hour = 3,600 kilojoule

1 kilojoule = 1,000 joule

How to convert kilowatt hour to kilojoule

To convert kilowatt hour to kilojoule, multiply the value by 3,600.

To convert kilojoule back to kilowatt hour, multiply by 0.00027778.

Measurement standards

The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.

Did you know?

A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.

Quick reference: kilowatt hour to kilojoule

kilowatt hour	kilojoule
0.1	360
0.5	1,800
1	3,600
2	7,200
5	18,000
10	36,000
25	90,000
50	180,000
100	360,000
250	900,000
500	1,800,000
1,000	3,600,000

Common values

	kilowatt hour	kilojoule
A food calorie (1 kcal)	0.00116222 kilowatt hour	4.184 kilojoule
A AA battery	0.0026 kilowatt hour	9.36 kilojoule
Boiling a kettle	0.11666667 kilowatt hour	420 kilojoule
Daily human diet (2000 kcal)	2.32444444 kilowatt hour	8,368 kilojoule
1 gallon of gasoline	36.6 kilowatt hour	131,760 kilojoule

Available Energy units

More kilowatt hour conversions

Assumption: calorie means thermochemical calorie, and therm means US therm.