The joule and the watt-hour both measure energy — they are the same physical quantity in two different units. Converting between them is one of the cleanest, most exact conversions the tool performs.
The exact identity
A watt is one joule per second. An hour is exactly 3600 seconds. So a watt-hour — one watt sustained for one hour — is exactly:
1 Wh = 3600 J1 kWh = 3.6 MJ
Follow the chain: 1 kWh = 1000 Wh = 1000 × 3600 J = 3,600,000 J = 3.6 MJ. Every step multiplies by a defined integer, so there is no rounding and no measurement anywhere.
Why this is exact, not approximate
This identity is exact because it follows entirely from definitions. The second is an SI base unit, the joule is defined from SI base units, the watt is one joule per second by definition, and the hour is exactly 3600 seconds by definition. Nothing here is measured or blended, so nothing varies. The tool labels this kind of result exact and shows it to full precision (capped only by the display setting), with no ~ marker — adding a tilde would imply a false doubt that does not exist.
Contrast that with a fuel question. "How much energy is in a cubic metre of natural gas?" also produces a number in joules or kilowatt-hours, but that number depends on the gas composition and is a sourced estimate, not an exact identity. The arithmetic joule↔watt-hour is exact; the content of a physical fuel is not.
Practical consequences
Because the factor is exact, you can round-trip freely: convert kWh to joules to megajoules and back with no drift. This is also why an electricity meter reading in kWh maps cleanly onto scientific energy units — the units differ, the energy does not. Just remember that a kilowatt-hour is energy, while a kilowatt is power; if you find yourself trying to convert a plain kilowatt into a kilowatt-hour, see kW vs kWh, because that step needs a duration and is not a unit conversion at all.