Methodology
This page explains exactly how EuroWattCalc's calculators arrive at the numbers they show, where default figures come from, and what each estimate deliberately does and does not include. We publish this methodology so that readers, journalists and fellow site owners can check our working rather than take a result on faith.
The core formula
Every running-cost calculator on this site is built on the same two-step physical relationship. First, power (watts) is converted into energy (kilowatt-hours) by multiplying by the time an appliance runs, then dividing by 1,000. Second, that energy figure is multiplied by a price per kilowatt-hour to produce a cost.
Cost: kWh × price per kWh = usage cost
This is standard electrical arithmetic, not a proprietary model — the same formula appears in physics textbooks and utility company explainer pages. What EuroWattCalc adds is a fast, appliance-specific interface with sensible defaults, plus the context needed to interpret the result correctly for a given appliance type.
Where default wattage figures come from
Where a calculator pre-fills a wattage value, that figure is chosen as a reasonable mid-range estimate for a common version of that appliance in a typical European household, based on manufacturer specification sheets, EU energy-label data and published efficiency guidance for that appliance category. Defaults are starting points, not measurements of your specific device — the single most accurate input you can provide is the wattage printed on your own appliance's rating plate, its manual, or a reading from a plug-in power meter.
How we handle average versus peak power
Some appliances — refrigerators, washing machines, tumble dryers and dehumidifiers among them — cycle their compressor or heating element on and off rather than drawing constant power. For these, we explain the difference between peak (nameplate) wattage and average running wattage on the relevant calculator page, and recommend using average wattage together with the appliance's total runtime for the most realistic estimate. Where an EU energy label states an annual kWh figure directly, that figure — divided by the appliance's typical operating hours — is usually the most reliable average wattage source available to a reader.
How tariffs and currency are handled
EuroWattCalc does not publish a single "European electricity price" because household tariffs vary enormously by country, supplier, contract type and time period, and a fixed continental average would mislead far more readers than it would help. Instead, every calculator asks you to enter your own unit price per kWh, sourced from a recent bill or your supplier's account. The currency selector changes only how the result is displayed; the underlying kWh and cost arithmetic is currency-neutral.
Official reference prices for the UK and France. Two exceptions use real, published prices so that answers like "what does a 2kW heater cost per hour" are concrete. UK pages use the Ofgem energy price cap: the Great Britain average unit rate and standing charge for a standard variable tariff paid by Direct Debit (currently 26.32p/kWh and 54.83p/day for 1 October to 31 December 2026). French pages use the regulated Tarif Bleu published in the CRE's open data (currently 0,2001 €/kWh in option Base). Every one of these figures comes from a single price file that is updated when the regulator publishes new rates, and each page shows the date the prices were last checked and a link to the source. They are still averages or reference tariffs: your own bill may differ.
What these calculators intentionally exclude
Household electricity bills typically include standing charges, network fees, taxes and sometimes time-of-use pricing structures on top of straightforward per-kWh usage cost. Our calculators estimate the variable, usage-based portion of a cost for a single appliance or scenario — they do not attempt to reconstruct your entire bill, since fixed charges are not caused by any single appliance and vary too much by supplier and country to estimate reliably without your actual contract.
Review and updates
UK prices are rechecked after each quarterly Ofgem price cap announcement, and French prices after each change to the regulated tariffs, usually in February and August.
Calculator pages, default figures and explanatory content are reviewed periodically and updated when formulas, terminology, typical appliance wattages or site functionality change materially. Our editorial policy describes the standards every page on this site is expected to meet.
Frequently asked questions
Is this the same formula an energy company would use?
Yes. Watts-to-kWh-to-cost is standard electrical engineering arithmetic used throughout the utility industry; there is no alternative "correct" formula, only differences in what additional bill components are layered on top.
Why do your default wattages sometimes differ from what I've read elsewhere?
Appliance wattage varies by brand, model, age and setting, so any published default is necessarily an approximation. We recommend always overriding the default with your own appliance's rated or measured power when you have it.
Do you test or verify appliance wattages yourselves?
Our defaults are drawn from manufacturer specifications, EU energy-label data and established efficiency guidance rather than in-house laboratory testing. We treat them as reasonable planning estimates, not certified measurements.
How often is this methodology reviewed?
We review calculator formulas and default figures periodically, and update this page whenever a material change is made to how a calculator works.