What is watt hour?
A watt hour is a unit of energy equal to the power of one watt sustained for one hour. It is used to measure small amounts of electrical energy consumption, particularly in battery capacities and portable electronics.
Real-world uses
Watt-hours measure the energy stored in or consumed from batteries, small electronics, and portable power stations. A smartphone battery holds roughly 10–15 Wh; a laptop battery 40–100 Wh. Solar panel output for small installations is often quoted in Wh per day.
History
The watt-hour emerged as a practical energy unit in the late 19th century with the commercialisation of electric power distribution. It offered a convenient scale between the joule (too small) and kilowatt-hour (too large) for everyday portable energy applications.
Common mistakes
Confusing watt-hours (energy) with watts (power). A device rated at 5 W running for 3 hours consumes 15 Wh. Also, treating Wh and mAh as interchangeable without knowing voltage: Wh = mAh × V ÷ 1000.
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?
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 watt hour = 3.6 kilojoule
1 kilojoule = 1,000 joule
How to convert watt hour to kilojoule
To convert watt hour to kilojoule, multiply the value by 3.6.
To convert kilojoule back to watt hour, multiply by 0.27777778.
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: watt hour to kilojoule
| watt hour | kilojoule |
|---|---|
| 0.1 | 0.36 |
| 0.5 | 1.8 |
| 1 | 3.6 |
| 2 | 7.2 |
| 5 | 18 |
| 10 | 36 |
| 25 | 90 |
| 50 | 180 |
| 100 | 360 |
| 250 | 900 |
| 500 | 1,800 |
| 1,000 | 3,600 |
Common values
| watt hour | kilojoule | |
|---|---|---|
| A food calorie (1 kcal) | 1.16222222 watt hour | 4.184 kilojoule |
| A AA battery | 2.6 watt hour | 9.36 kilojoule |
| Boiling a kettle | 116.66666667 watt hour | 420 kilojoule |
| Daily human diet (2000 kcal) | 2,324.44444444 watt hour | 8,368 kilojoule |
| 1 gallon of gasoline | 36,600 watt hour | 131,760 kilojoule |
Available Energy units
More watt hour conversions
- Convert watt hour to joule
- Convert watt hour to kilojoule
- Convert watt hour to kilowatt hour
- Convert watt hour to calorie (thermochemical)
- Convert watt hour to kilocalorie (dietary)
- Convert watt hour to megajoule
- Convert watt hour to electronvolt
- Convert watt hour to British thermal unit
- Convert watt hour to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.