One kilowatt-hour is exactly 3,600,000 joules, or 3.6 megajoules. The number comes straight from the definition of the units: a watt is a joule per second, and an hour is 3,600 seconds.
About the units
The joule is the SI unit of energy, defined as the work done when a force of one newton moves through one metre. It is small in human terms — lifting an apple one metre takes about one joule — which is why practical energy quantities use prefixes or entirely different units. The kilowatt-hour is one of those: it is not an SI unit but a product of a power and a time, and it exists because electricity is billed for a rate of delivery over a period. A device drawing one kilowatt for one hour consumes one kilowatt-hour, which makes the unit intuitive for anyone reading a meter.
The exact factor
The derivation is 1,000 W × 3,600 s = 3,600,000 J. Both directions are exact. Useful anchors: a kettle boiling for three minutes uses about 0.15 kWh, or 540 kJ. An electric car with a 60 kWh battery stores 216 megajoules. A joule per second is a watt, so any power figure multiplied by a duration in seconds gives joules directly — which is the calculation that connects the two units in every energy problem.
Where you meet this conversion
Electricity billing everywhere, and increasingly heat as well, since district heating and heat pumps are metered in kilowatt-hours. Battery capacity for vehicles and home storage is quoted in kilowatt-hours while cell specifications are in watt-hours or ampere-hours. Scientific and engineering work uses joules, so any calculation that starts from a utility bill and ends in a physics equation crosses this boundary. National energy statistics use terawatt-hours or petajoules depending on the publisher, and the two are related by this same factor scaled up.