Kilowatt-hours to megajoules – and megajoules to kilowatt-hours

Formula

Multiply the value in kilowatt-hours by 3.6 to get megajoules.
MJ = kWh × 3.6

Multiply the value in megajoules by 0.27777778 to get kilowatt-hours.
kWh = MJ × 0.27777778

How to convert kilowatt-hours to megajoules

Type a value in the field above and the result is calculated instantly. Use the arrow button to swap direction, or pick other units from the lists.

Doing it in your head

A rough conversion for when the calculator is not to hand. The error comes last, and it holds in both directions at that order of magnitude.

• Megajoules to kilowatt-hours: ÷ 4 + 10%. That gives 0.275 instead of 0.277778, an error of 1%.

About the units

Kilowatt-hours. A kilowatt-hour is the energy of one kilowatt for one hour, exactly 3.6 megajoules. It needs no definition of its own: the watt and the hour are both legal units in the EU, and combinations of legal units are legal too. It is the unit on the electricity bill and on the EU energy label, where a fridge’s annual consumption is given as a whole number of kilowatt-hours per year. The trap is kW against kWh: the kilowatt is power, the kilowatt-hour energy.

Megajoules. A megajoule is a million joules, since the prefix mega means 10⁶, and as a decimal multiple of the joule a legal unit throughout the EU. It is used for the energy content of fuels, for technical energy balances and for raw food data. The simple anchor is that one kilowatt-hour is exactly 3.6 megajoules. Two traps: megajoules and megawatt-hours are a factor of 3,600 apart, and megajoules per kilogram is not megajoules per litre, since oil and diesel differ in density.