Use kW, hours and kWh correctly and rearrange the relationship for runtime.
Core idea
Power is the rate at which energy is transferred. Energy equals power multiplied by time. With power in kilowatts and time in hours, the result is kilowatt-hours: energy (kWh) = power (kW) × time (h). Runtime equals energy divided by power.
What to recognise
- Convert watts to kilowatts by dividing by 1000.
- Convert minutes to hours by dividing by 60.
- A 3 kW heater running for 40 minutes uses 3 × 40/60 = 2 kWh.
- Two identical loads running together double power, not time.
- Use actual duty cycle or measured input when the appliance does not run continuously.
Apply it
Write the units beside every value before multiplying. For a 6 kWh energy requirement supplied by a constant 2 kW heater, runtime is 6 ÷ 2 = 3 hours. Sense-check: a larger power should meet the same energy requirement in less time.
Keep the boundary clear
This simple relationship assumes the stated input remains constant. Thermostats, cycling, defrost, standby loads and changing output affect real consumption. Electrical input and useful heat output are not the same for a heat pump, so choose the quantity the question actually asks for.
Something wrong, unclear or out of date? Report an issue with this note.