Watts, kilowatts and kWh explained

Power and energy are not interchangeable

A 2,000 W appliance is rated at 2 kW. If it draws that continuously for half an hour, it uses 1 kWh. The same energy could be used by a 500 W device over two hours. Time is essential to a fair cost comparison.

Keep units attached to every number. Watts describe power, while kWh describe energy. The calculators apply the unit rate in pence per kWh and show pounds or pence without hiding the conversion.

The basic conversion

Divide watts by 1,000 to get kilowatts, then multiply by hours. Minutes are divided by 60 first. Finally multiply kWh by pence per kWh and divide by 100 to express the cost in pounds.

A short-use example

A 3,000 W kettle running for five minutes uses 3 × 5/60 = 0.25 kWh if it draws full power for that whole period. At 25p/kWh this is 6.25p. Actual boil time changes with the water quantity and temperature.

Real use can vary by model, settings, condition and household routine. Test more than one reasonable scenario when a single assumption drives the answer. That range is more useful than reporting an over-precise total.

Why labels differ

Some labels give input watts, others kWh per cycle or annual kWh. Use the mode matching the available figure. Do not put cooling capacity, heat output, battery capacity or microwave cooking output into a field asking for electrical input.

From one use to a year

Frequency turns a single-use estimate into an annual one. Daily use is multiplied by 365, weekly by 52 and monthly by 12. The calculator then divides the annual figure consistently to display typical weeks and months.

Applying this guide to your household

Start with the best source available: the product’s electrical input, an energy-label kWh value, the manual, or a safe representative measurement. Match the unit and period in the calculator. Enter your own tariff rather than a quoted national average, and keep the standing charge separate.

Check the live calculation breakdown after submitting. It repeats your inputs and shows how energy becomes cost. Save clearly labelled results to the basket, where you can change the tariff and compare each item’s share of the saved total. The basket does not send the data to this website.

If a comparison involves purchasing equipment, separate energy arithmetic from the financial decision. Purchase, delivery, installation, disposal, maintenance and ownership period can change payback. Repairability, suitability and safety are also relevant even though they are not converted into money here.

Checking whether the result is reasonable

Sense-check the order of magnitude before acting. A very high wattage used for only a few minutes may consume less energy than a modest load left running all day. Compare the calculated annual kWh with the period and frequency entered, and make sure pence were not entered as pounds or vice versa.

Run a low, central and high scenario when duration or cycling is uncertain. Record why each assumption was chosen. If a monitored figure is available, measure a complete representative programme or several ordinary days rather than selecting an unusually light session. Seasonal equipment needs observations from conditions similar to those being estimated.

Finally, distinguish the appliance estimate from the household bill. The bill can include every electrical load, standing charge, tariff changes, corrections and account adjustments. A difference does not automatically mean the formula is wrong; first compare the same time period, tariff basis and set of loads.

Limitations and assumptions

Results are estimates based on the information entered. Actual energy use can vary by appliance model, settings, temperature, cycling, condition and household behaviour. The calculation cannot predict future tariffs, repairs or behavioural changes. It estimates electricity only and does not include gas, water, detergent or the daily electricity standing charge unless a page explicitly says otherwise.

Examples explain the maths and are not claims about every appliance. This information is general, not electrical, installation, medical or financial advice. Follow manufacturer instructions and obtain appropriately qualified help where a safety-critical decision requires it.