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 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.
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 watt hour = 3,600 joule
1 joule = 1 joule
How to convert watt hour to joule
To convert watt hour to joule, multiply the value by 3,600.
To convert joule back to watt 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: watt hour to joule
| watt hour | joule |
|---|---|
| 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
| watt hour | joule | |
|---|---|---|
| A food calorie (1 kcal) | 1.16222222 watt hour | 4,184 joule |
| A AA battery | 2.6 watt hour | 9,360 joule |
| Boiling a kettle | 116.66666667 watt hour | 420,000 joule |
| Daily human diet (2000 kcal) | 2,324.44444444 watt hour | 8,368,000 joule |
| 1 gallon of gasoline | 36,600 watt hour | 131,760,000 joule |
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.