Energy is the capacity to do work. Lifting a weight, heating a room, driving a motor, powering a laptop — physically these are all the same quantity being moved around and transformed. Because it is one quantity, it can be expressed in many interchangeable units: joules, watt-hours, calories, British thermal units, and larger industrial units built on top of those.
One quantity, many units
The SI unit of energy is the joule (J). Every other energy unit is defined against it by a fixed factor. A watt-hour is exactly 3600 J; a kilowatt-hour is exactly 3.6 MJ. These are not measurements or approximations — they are definitions, so converting between them is exact. See joule vs watt-hour for why that particular identity is exact rather than merely close.
Because the conversions are definitional, the tool marks pure energy-unit conversions as exact. No source is needed beyond the definition itself, and no material assumption enters the calculation. You can move a value freely between joule, kilowatt-hour, megajoule and the rest without losing anything.
Energy units are exact; fuel content is not
There is a crucial distinction the tool never blurs. Converting between energy units is exact. But asking how much energy is in a physical thing — a litre of diesel, a cubic metre of natural gas, a kilogram of wood — is a different question entirely. That answer depends on a material property (a heating value) that genuinely varies with blend, composition, moisture and temperature. Those results are sourced estimates, not exact identities.
So 1 kWh = 3.6 MJ is exact forever, but "1 litre of diesel = X kWh" is a representative, sourced figure with its assumptions on display. Keeping these two kinds of answer visibly separate is a core design rule; read why fuel conversions are approximate for the reasoning.
What the tool does with this
Every result carries an exactness label so you always know which kind of answer you are looking at: an exact unit identity, a value fixed by convention, a sourced estimate, or a figure that needs more context before it can be computed at all. When a number comes from data rather than a definition, its provenance is one click away on the sources panel. The goal is never to hide uncertainty behind a confident-looking number — an energy converter that conceals its assumptions is worse than useless.