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 megajoule?
A megajoule is a unit of energy equal to one million joules. It is used in industrial energy calculations, explosive yield estimations, and large-scale thermodynamic systems.
Real-world uses
Megajoules are used in energy analysis for fuel content (1 litre of petrol contains about 34.2 MJ), building energy ratings, and industrial process energy requirements. Life cycle assessment (LCA) studies report embodied energy in MJ per unit of material.
History
The megajoule became a standard unit in energy engineering and policy analysis during the 20th century. It provides a convenient scale for comparing fuel energy content, industrial processes, and building energy performance across different energy sources.
Common mistakes
Confusing MJ with MWh—1 MWh = 3,600 MJ. Also, when comparing fuel energy densities, failing to distinguish between lower heating value (LHV) and higher heating value (HHV), which can differ by 5–10% for hydrocarbon fuels.
When is this conversion used?
Converting watt hour to megajoule is useful in the energy domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 watt hour = 0.0036 megajoule
1 megajoule = 1,000,000 joule
How to convert watt hour to megajoule
To convert watt hour to megajoule, multiply the value by 0.0036.
To convert megajoule back to watt hour, multiply by 277.77777778.
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 megajoule
| watt hour | megajoule |
|---|---|
| 0.1 | 0.00036 |
| 0.5 | 0.0018 |
| 1 | 0.0036 |
| 2 | 0.0072 |
| 5 | 0.018 |
| 10 | 0.036 |
| 25 | 0.09 |
| 50 | 0.18 |
| 100 | 0.36 |
| 250 | 0.9 |
| 500 | 1.8 |
| 1,000 | 3.6 |
Common values
| watt hour | megajoule | |
|---|---|---|
| A food calorie (1 kcal) | 1.16222222 watt hour | 0.004184 megajoule |
| A AA battery | 2.6 watt hour | 0.00936 megajoule |
| Boiling a kettle | 116.66666667 watt hour | 0.42 megajoule |
| Daily human diet (2000 kcal) | 2,324.44444444 watt hour | 8.368 megajoule |
| 1 gallon of gasoline | 36,600 watt hour | 131.76 megajoule |
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.