Convert Watt-Hour to Megajoule (Wh → MJ)

Converting watt-hours to megajoules is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.

1. 1
   Take your value in watt-hours

   Start with the number of watt-hours (Wh) you want to convert.
2. 2
   Multiply by 0.0036

   The conversion factor from Wh to MJ is 0.0036. Multiply your value by this number.
3. 3
   Read the result in megajoules

   The result is your value in megajoules (MJ).

Multiply the value in watt-hours by 0.0036. For the reverse direction, multiply by 277.77778.

• ✓1 Wh = 3600 J = 0.001 kWh.
• ✓Battery mAh × V = Wh. A 3000 mAh phone battery at 3.7 V = 11.1 Wh.
• ✓Airline limits lithium batteries to 100 Wh for carry-on.

• ✗Confusing mAh with Wh — need voltage to convert.
• ✗Using Wh when kWh is more appropriate for large batteries.
• ✗Forgetting the voltage in battery capacity calculations.

What is the Watt-Hour?

The watt-hour equals exactly 3,600 joules and is the standard unit for small-battery capacity, household electrical energy, and renewable-energy storage. A smartphone battery stores roughly 12–15 Wh, a laptop battery 50–100 Wh, and an electric car battery 60,000–100,000 Wh (60–100 kWh). The watt-hour represents the energy delivered by a 1-watt device running for 1 hour. It is the natural unit for connecting power (watts) to time, which is why electric utilities bill in kilowatt-hours. The watt-hour relates to the joule (3,600 J = 1 Wh), the kilowatt-hour (1,000 Wh = 1 kWh), and the BTU (1 Wh ≈ 3.412 BTU). Battery capacity, solar panel output (Wh per day), and energy storage system specifications all rely on the watt-hour as the base small-energy unit.

• Laptop and phone battery capacity
• Portable power banks and UPS
• Home solar panel daily output

What is the Megajoule?

The megajoule equals 1,000,000 joules (or 1,000 kilojoules) and is used for measuring fuel energy content, large-scale electrical work, and industrial heat applications. The energy content of food and fuel is often quoted in MJ: a kilogram of gasoline contains about 46 MJ, natural gas has 38 MJ per cubic meter, and a kilogram of coal yields 24 MJ. Vehicle fuel-economy and energy-efficiency calculations use MJ to compare disparate energy sources. Industrial process heating, boiler outputs, and large machinery energy ratings are commonly given in MJ per cycle or MJ per hour. The megajoule relates to the joule (10⁶ J = 1 MJ), the kilojoule (1,000 kJ = 1 MJ), the kilowatt-hour (1 kWh = 3.6 MJ), and the BTU (1 MJ ≈ 948 BTU). For electricity-grid energy accounting, the equivalent kilowatt-hour or megawatt-hour is more common than MJ.

• Fuel energy content reporting
• Natural gas billing in some countries
• Industrial energy consumption