The single most common mistake in energy arithmetic is treating power as if it were energy — reading 1 kW as though it meant 1 kWh. They are different physical quantities, and the tool will not silently convert one to the other.
Power is a rate; energy is an amount
Power (watts, kilowatts, megawatts) is a rate — how fast energy is delivered or consumed at an instant. Energy (joules, watt-hours, kilowatt-hours) is an amount — the total moved over some span of time. A 2 kW heater running for 3 hours consumes 6 kWh; the same heater tells you nothing about total energy until you also say for how long.
The relationship is simply energy = power × time (E = P · t). Given any two of the three you can find the third, but you genuinely need two. See joule vs watt-hour for how the energy side of that equation is built.
Why the tool refuses kW → kWh without a duration
Converting kilowatts to kilowatt-hours is a cross-dimension bridge from power to energy, and that bridge requires a time input. Without one, the tool does not guess and does not fail — it returns a context required result and offers you a time field. This is deliberate: "1 kW → kWh with no time" is a well-defined question the moment you add a duration, so the honest response is to ask for it rather than fabricate a number.
Once you supply a time, the arithmetic itself is exact: 2 kW × 3 h = 6 kWh. The displayed precision is still bounded by how precise your inputs were, but the multiplication introduces no material assumption.
Defining "year" for time arithmetic
Time arithmetic hides an ambiguity: how long is a "year"? Silently using 365, 365.25 or 360 days would make results irreproducible. The tool fixes the Julian year = 365.25 days = 31,557,600 s and labels it, so a conversion like average power over a year is documented and repeatable. An hour is likewise exactly 3600 s. These time definitions are exact; it is only the crossing between power and energy that needs your duration.