Megajoules to kilowatt hours
One megajoule is 0.277777777778 kilowatt hours, and this shows the working rather than just the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The factor
One megajoule is 0.277777777778 kilowatt hours. That is the whole of the conversion: multiply by it, and nothing else happens.
And it is exact
Not rounded, not measured, not a convention that might be revised. The factor above is the definition, so an answer from it is as exact as the number you typed.
The kilowatt hour is exactly 3.6 million joules. What an electricity bill counts.
The number in your head
Most people use 0.28 for this, which is the factor to two significant figures. It is high by 0.8% — 8 kilowatt hours too many in every thousand, and 80 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 3.6, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 3.6, which is high by 1.233581e-14%. So the two roundings do not cancel: go out and back with both and you do not land where you started.
How many of those decimals mean anything
The box above will give you ten decimal places because it can. Whether they mean anything is decided by your measurement, not by the arithmetic.
If you measured to the nearest megajoule, your figure is already uncertain by half of one — so the answer is uncertain by about 0.138889 kilowatt hours, and every digit past that is decoration. Quoting 0.34293333 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Megajoules and kilowatt hours both measure energy, which is why they can be converted at all. A conversion between families — a length into a weight — is not a conversion, it is a question with a missing quantity in it.