watt hour (Wh)
1 watt hour = 3,600 joules
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.
Notes: Electrical energy
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.
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.
How to convert watt hour to joule
To convert watt hour to joule, multiply the value by 3,600.
Conversion formula (from base unit)
x * 3600.0
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.
| watt hours | joules |
|---|---|
| 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 hours | joules | |
|---|---|---|
| A food calorie (1 kcal) | 1.16222222 watt hours | 4,184 joules |
| A AA battery | 2.6 watt hours | 9,360 joules |
| Boiling a kettle | 116.66666667 watt hours | 420,000 joules |
| Daily human diet (2000 kcal) | 2,324.44444444 watt hours | 8,368,000 joules |
| 1 gallon of gasoline | 36,600 watt hours | 131,760,000 joules |
Full watt hour conversion table
| Unit | Value | Factor |
|---|---|---|
| joule | 1 Wh = 3,600 J | × 3,600 |
| kilojoule | 1 Wh = 3.6 kJ | × 3.6 |
| kilowatt hour | 1 Wh = 0.001 kWh | × 0.001 |
| calorie (thermochemical) | 1 Wh = 860.4206501 cal | × 860.4206501 |
| kilocalorie (dietary) | 1 Wh = 0.86042065 kcal | × 0.86042065 |
| megajoule | 1 Wh = 0.0036 MJ | × 0.0036 |
| electronvolt | 1 Wh = 2.246943e+22 eV | × 2.246943e+22 |
| British thermal unit | 1 Wh = 3.41214163 BTU | × 3.41214163 |
| therm (US) | 1 Wh = 0.00003413 therm | × 0.00003413 |