Rates last reviewed: October 2026. UK rates change every quarter; US rates are reviewed every quarter.
This page explains exactly how PlugPenny calculates running costs: the formula, the official rates we use, how we measure appliances and what our estimates leave out. If anything here is unclear or wrong, tell us.
1. The formula
Electricity is billed in kilowatt-hours (kWh). One kWh is the energy used by a 1,000-watt appliance running for one hour.
Energy (kWh) = power (watts) ÷ 1,000 × hours of use
Cost = energy (kWh) × price per kWh
Example: a 1,500 W space heater running for 8 hours uses 1.5 × 8 = 12 kWh. At 18.2¢ per kWh that is $2.18 a day. At 26.32p per kWh it is £3.16 a day.
Monthly costs multiply the daily cost by the days of use you choose (30 by default). Yearly costs multiply the monthly cost by 12.
2. The rates we use
| Market | Default rate | Source | What it represents |
|---|---|---|---|
| United States | 18.2¢ per kWh | U.S. Energy Information Administration, Short-Term Energy Outlook (EIA Short-Term Energy Outlook, 2026 average residential price) | A national average. The EIA calculates it from utilities’ residential revenue and sales, so it roughly spreads energy, delivery and fixed charges across each kWh. Your local rate can be much higher or lower. |
| United Kingdom (Great Britain) | 26.32p per kWh, plus a standing charge of 54.83p per day | Ofgem energy price cap (Ofgem price cap, 1 October to 31 December 2026 (GB average, Direct Debit, 0% VAT)) | The average maximum unit rate for electricity on a default (standard variable) tariff paid by Direct Debit in England, Scotland and Wales. Domestic electricity carries 0% VAT from 1 October 2026 to 31 March 2027, so the cap figures for that period include no VAT. Rates vary by region and payment method; the cap does not apply to Northern Ireland or to fixed tariffs. |
US state rates. When we quote a state, we use the latest monthly average residential price published by the EIA in its Electric Power Monthly, and we show the month the figure refers to.
Your own tariff is always better than an average. Every calculator lets you type the price per kWh shown on your bill.
3. Where appliance wattage comes from
We use, in this order of preference:
- Our own measurements (section 4), marked “We measured it” with a photo of the meter.
- Official efficiency data, such as ENERGY STAR listings in the US and energy labels in the UK, which give annual kWh for fridges, freezers, washers and dryers.
- Manufacturer specifications from the rating plate or the manual.
- Typical ranges when the model is unknown, stated as a range and labelled “typical”.
Some appliances do not draw full power all the time. Fridges, freezers, ovens, heaters with thermostats, air conditioners and hot tubs cycle on and off. For these we use the measured or published average power over a day, or we explain how the result changes with the share of time the appliance is actually running.
4. How we measure
- Equipment: a plug-in energy meter that records watts and cumulative kWh.
- Duration: at least 24 hours for appliances that cycle; a full programme for washers, dryers and dishwashers; repeated short runs for kettles, microwaves and air fryers.
- Recording: we note the appliance model, settings, room temperature and date, and photograph the meter display.
- Calculation: we divide the kWh recorded by the hours of the test to get an average, then apply the rates in section 2.
- Limits: plug-in meters only work with appliances that plug into a standard socket and must be used within their rated current. We never measure hard-wired appliances (central air conditioning, electric water heaters, built-in ovens, EV chargers) ourselves; for those we use official and manufacturer data.
5. What our estimates do not include
- Standing and fixed charges, unless a calculator lets you add them. In the UK they are paid every day whether or not you use electricity.
- Time-of-use and tiered rates. If your price changes by time of day or by how much you use, enter the rate that applies to the hours you use the appliance.
- Taxes, surcharges and local fees that are not already part of the average rate.
- Gas costs. Our electricity calculators cover electricity only; gas comparisons say so explicitly and use the relevant gas rate.
6. Rounding
We calculate with full precision and round only the displayed result: to the nearest cent or penny for amounts under one dollar or pound, and to two decimal places for larger amounts. Monthly and yearly figures in tables may differ slightly from the sum of rounded daily figures.
7. How we keep figures current
- UK: we update the rate every time the Ofgem price cap changes (1 January, 1 April, 1 July and 1 October).
- US: we review the national and state averages every quarter, using the latest EIA publications.
- Each article shows the date its figures were last checked. We only change that date when we actually review the content.
8. Corrections
If you find an error, contact us with the page address. We verify reports, fix confirmed errors promptly and add a correction note to the article when a change affects its conclusions.