How we calculate

Last updated 30 October 2026

Every running cost on WattCosts comes from one formula, one official price and the appliance's own power rating. Here is each part, so you can check any number yourself.

The formula

Cost = power in watts ÷ 1,000 × hours used × price per kWh.

Dividing watts by 1,000 gives kilowatts. Kilowatts multiplied by hours gives kilowatt-hours (kWh), the unit on your bill. A 2,000W heater on for one hour uses 2 kWh. At 26.32p per kWh that costs 52.6p.

The price we use

We use the Ofgem energy price cap: 26.32p per kWh for electricity from 1 October to 31 December 2026. Average for Direct Debit customers in England, Scotland and Wales. No VAT on domestic electricity from 1 October 2026 to 31 March 2027. Gas prices include 5% VAT.

The cap is the most widely used benchmark because most UK households on a standard variable tariff pay it, and fixed tariffs are usually priced close to it. Your own rate can differ by region, payment method and tariff, so every calculator lets you type in the price from your bill. See Ofgem's published rates.

Ofgem reviews the cap every three months. It will announce the rate for 1 January to 31 March 2027 on or before 25 November 2026, and we update every page on this site the same day.

What we leave out

Running costs on this site cover the electricity each appliance uses. They do not include the daily standing charge (54.83p a day at the current cap), because you pay that whether or not the appliance is switched on.

Power ratings

• Model pages use the rating from the manufacturer's spec sheet, manual or official product page, and link to it. Where a model has several settings, we show each one.
• Appliance pages (for example "oil-filled radiators") show a range of common ratings, so you can find yours on the label and pick the closest.
• The rating printed on the appliance is its maximum draw. If yours is different from ours, use your figure in the calculator.

Thermostats and cycling

Many appliances do not draw full power the whole time they are on. A heater with a thermostat switches its element off once the room is warm, a fridge's compressor runs in bursts, and a dehumidifier slows down as the air dries. For these, the calculator has a "share of time at full power" setting. We suggest a starting figure on each page and explain where it comes from; change it if you know how your appliance behaves.

Rounding

We calculate with full precision and round only the figure you see: to two decimal places under 10p, one decimal place under £1, and to the penny above that.

How pages are written

Drafts are prepared with the help of AI writing tools. Owais Farooq checks every figure against its source and tests each calculator before a page is published. Each page shows the date it was last updated.

Corrections

If you find an error, email hello@wattcosts.co.uk. Confirmed corrections are made promptly and noted on the page.