Megajoules to Watt Hours
Type a value in either box to convert between megajoules and watt hours. The factor is 277.778.
| Megajoules | Watt Hours |
|---|---|
| 1 MJ | 277.777778 Wh |
| 2 MJ | 555.555556 Wh |
| 3 MJ | 833.333333 Wh |
| 5 MJ | 1388.888889 Wh |
| 10 MJ | 2777.777778 Wh |
| 25 MJ | 6944.444444 Wh |
| 50 MJ | 13888.888889 Wh |
| 100 MJ | 27777.777778 Wh |
How to use
- Enter a value in the Megajoules field.
- The Watt Hours figure appears immediately — no button to press.
- Typing in the Watt Hours field converts back to Megajoules the same way.
- Use the reference table for the values that come up most often.
Megajoules to Watt Hours formula
The factor from megajoules to watt hours is 277.777778.
Wh = MJ × 277.777778
Why this factor: the megajoule and the watt-hour are both defined against the joule, so dividing one definition by the other gives 277.777778 directly. Both definitions are set out in full below.
To go the other way, divide by 277.777778 — or use the Watt Hours to Megajoules converter, which is the same calculation reversed.
Worked example
Given 500 MJ, multiplying by 277.777778 yields 138888.888889 Wh.
500 MJ × 277.777778 = 138888.888889 Wh
The converter above does this for any value you type, in either direction.
What is a megajoule?
A megajoule is one million joules, used for substantial energy quantities.
Definition: Exactly 1×10⁶ joules. One kilowatt-hour is exactly 3.6 MJ.
Where it is used: Fuel energy content, natural gas billing, and industrial energy accounting.
What is a watt-hour?
A watt-hour is the energy delivered by one watt of power sustained for one hour.
Definition: Exactly 3,600 joules.
Where it is used: Battery capacity for laptops, e-bikes and power tools, and small-scale energy metering.
Worth knowing: Airline lithium-battery limits are set in watt-hours, which is why laptop and power-bank capacities are labelled in Wh.
Megajoules vs Watt Hours
The megajoule is a metric unit while the watt-hour is a specialized unit. That is why the factor between them is not a round number: the two systems were defined independently and only later tied together by exact agreement.
Frequently asked questions
How many watt hours are in a megajoule?
One megajoule equals 277.777778 watt hours.
What is the formula to convert megajoules to watt hours?
Multiply the number of megajoules by 277.777778. To reverse it, divide by 277.777778 instead.
How do I convert watt hours back to megajoules?
Divide by 277.777778, which is the same as multiplying by 0.0036.
How many watt hours are in 10 megajoules?
10 MJ works out to 2777.777778 Wh.
Is there anything to watch out for with watt hours?
Airline lithium-battery limits are set in watt-hours, which is why laptop and power-bank capacities are labelled in Wh.
How is the megajoule defined?
Exactly 1×10⁶ joules. One kilowatt-hour is exactly 3.6 MJ.