What is joule?
The joule is the SI base unit of energy. It is the standard unit for work, heat, and electrical energy in physics, engineering, and scientific measurement worldwide.
Real-world uses
The joule is the SI unit of energy used in physics, chemistry, and nutrition (food energy in some countries is labelled in kilojoules). Electrical energy, mechanical work, and heat transfer are all measured in joules. One joule is the energy to lift a small apple about 1 metre.
History
Named after James Prescott Joule, the English physicist who demonstrated the mechanical equivalent of heat in the 1840s, establishing that heat and mechanical work are interchangeable forms of energy. The joule was adopted as the SI energy unit in 1960.
Common mistakes
Confusing joules with watts—a joule is a unit of energy, while a watt is a unit of power (1 W = 1 J/s). Also, assuming "calorie" and "joule" are equivalent without conversion: 1 cal = 4.184 J.
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.
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.
Worked examples
1 joule = 2.777778e-07 kilowatt hour
1 kilowatt hour = 3,600,000 joule
How to convert joule to kilowatt hour
To convert joule to kilowatt hour, multiply the value by 2.777778e-07.
To convert kilowatt hour back to joule, multiply by 3,600,000.
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: joule to kilowatt hour
| joule | kilowatt hour |
|---|---|
| 0.1 | 2.777778e-08 |
| 0.5 | 1.388889e-07 |
| 1 | 2.777778e-07 |
| 2 | 5.555556e-07 |
| 5 | 0.00000139 |
| 10 | 0.00000278 |
| 25 | 0.00000694 |
| 50 | 0.00001389 |
| 100 | 0.00002778 |
| 250 | 0.00006944 |
| 500 | 0.00013889 |
| 1,000 | 0.00027778 |
Common values
| joule | kilowatt hour | |
|---|---|---|
| A food calorie (1 kcal) | 4,184 joule | 0.00116222 kilowatt hour |
| A AA battery | 9,360 joule | 0.0026 kilowatt hour |
| Boiling a kettle | 420,000 joule | 0.11666667 kilowatt hour |
| Daily human diet (2000 kcal) | 8,368,000 joule | 2.32444444 kilowatt hour |
| 1 gallon of gasoline | 131,760,000 joule | 36.6 kilowatt hour |
Available Energy units
More joule conversions
- Convert joule to kilojoule
- Convert joule to kilowatt hour
- Convert joule to calorie (thermochemical)
- Convert joule to kilocalorie (dietary)
- Convert joule to watt hour
- Convert joule to megajoule
- Convert joule to electronvolt
- Convert joule to British thermal unit
- Convert joule to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.