A kilowatt is a rate; a kilowatt-hour is an amount. A 2 kW kettle running for six minutes uses 2 × 0.1 = 0.2 kWh, which at the Great Britain price cap average of 26.32 pence per kWh for 1 October to 31 December 2026 costs 5.3 pence (Ofgem's unit rates and standing charges). The condition that changes the sum is constancy: the arithmetic only holds while the appliance actually draws its stated power for the whole of the stated time.

The conversion that goes wrong

One kilowatt is 1,000 watts. To turn watts and minutes into kilowatt-hours, both units have to move before they are multiplied, and the mistake almost everyone makes is multiplying one converted number by one unconverted one.

Take a 600 W appliance running for 20 minutes. Convert the power: 600 ÷ 1,000 = 0.6 kW. Convert the time: 20 ÷ 60 = 0.3333 hours. Multiply: 0.6 × 0.3333 = 0.2 kWh. The same answer arrives through watt-hours: 600 × 0.3333 = 200 Wh, and 200 ÷ 1,000 = 0.2 kWh. What you must not do is multiply 0.6 by 20 and call the result kWh; the 20 is still minutes, so the answer is 12 kilowatt-minutes, which is 0.2 kWh wearing a disguise.

Split the two jobs between you. One reads the numbers off the rating plate or the manual; the other says out loud which unit each number is in before anything is multiplied. Swap after two appliances.

Three ways to reach the same 0.5 kWh

Power alone never tells you how much energy something used. These three cases, all assuming constant draw, land on the same amount and therefore the same cost at the October to December 2026 cap rate.

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Constant powerTimeEnergyCost at 26.32p per kWh
100 W = 0.1 kW5 hours0.5 kWh13.2p
500 W = 0.5 kW1 hour0.5 kWh13.2p
1,000 W = 1 kW30 minutes = 0.5 hours0.5 kWh13.2p

Each cost is 0.5 × 26.32p = 13.16p, rounded to 13.2p, using the cap average for 1 October to 31 December 2026 (Ofgem). The 1 kW appliance is ten times the rate of the 100 W one and uses exactly the same energy, because it runs for a tenth of the time. That is the whole distinction, and it is why a label showing a big wattage settles nothing on its own — a point worth carrying into reading an energy label together.

What the rate turns into on a bill

Now run the sum on something that runs all year. The European Commission puts the average electricity consumption of refrigerator-freezers sold in the EU in 2020 at 181 kWh a year, against 477 kWh a year for models sold in 1990 (the Commission's page on fridges and freezers).

  • At the Great Britain cap rate: 181 × 26.32p = 4,763.92p, or £47.64 a year.
  • At the EU average household electricity price of €0.2896 per kWh in the second half of 2025, a weighted average including taxes and VAT for households using 2,500 to 5,000 kWh a year (Eurostat's electricity price statistics): 181 × €0.2896 = €52.42 a year.

Two currencies, two markets, one appliance figure that belongs to neither of them on its own — the 181 kWh is an EU average for models sold in 2020, not a measurement of the fridge in your kitchen. Name all three when you write the answer down, and the number stays usable a year later.

The formula also has a limit worth stating once: it assumes the stated power is drawn throughout. A rated or maximum power on a label does not establish that, and an appliance that cycles between modes — a fridge, a washing machine, anything with a thermostat — needs a measured energy total or the manufacturer's declared annual figure instead. That is exactly why energy labels print kWh a year or kWh per 100 cycles rather than watts.

Write the three boxes on the sheet in this order: power, time, energy. Copy the unit next to every number. If a statement already gives kWh for a month, it is an amount, and multiplying it by hours again answers a question nobody asked — which is where why a lower bill does not always mean less energy picks the story up, and where a home energy observation gets you the records to try it on.

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