The short answer
kW measures power, the rate of electricity use; kWh measures energy used over time. A constant 1 kW draw for one hour uses 1 kWh. Convert watts to kW by dividing by 1,000, then multiply by hours to obtain energy. Electricity cost uses kWh multiplied by your price per kWh.
Power and energy describe different things
kW (kilowatts) measures power: the rate at which electricity is used. kWh (kilowatt-hours) measures energy: how much electricity is used over time.
For a device drawing a constant amount of power:
Energy in kWh = power in kW × time in hours
If the power rating is in watts, divide by 1,000 to express it in kilowatts.
An illustrative calculation
A device drawing 1 kW continuously for 2 hours uses 2 kWh. At an illustrative electricity price of £0.28 per kWh, that energy costs £0.56.
The constant-power assumption matters. A fridge’s compressor switches on and off; multiplying its rated watts by every hour in the year can produce an unsuitable estimate.
Choose a consumption figure
For a fridge, start with an appropriate annual kWh figure from the label or a measurement. For a cycle-based appliance, use the stated kWh per cycle or kWh per 100 cycles and enter your frequency.
The label-based calculators use the selected energy basis and your tariff. The separate watts-and-hours calculator estimates a suitable constant-power routine or accepts measured energy over complete days. Neither approach infers an appliance’s duty cycle from its nameplate.
Open the running-cost calculators or read about energy per 100 cycles.
Check which number your question needs
| Question | Relevant quantity |
|---|---|
| How quickly is electricity being used at this moment? | Power in W or kW |
| How much was used during a completed measurement? | Accumulated energy in kWh |
| What does that energy cost? | kWh multiplied by the matching tariff |
| How much will a household routine use? | Compatible per-task energy and task frequency |
For example, a 500 W constant draw is 0.5 kW. Across three hours it uses 1.5 kWh, costing £0.42 at the illustrative £0.28 rate. If it switches on and off, you need a suitable accumulated measurement or an established average rather than assuming the full rating applies throughout.
Choose an input that matches the tool: annual or per-cycle kWh for a label-based estimate, or electrical input and compatible use time for the watts-and-hours tool. Read the measurement guide if you want to use accumulated kWh instead of a constant-power assumption.