How we work

Last updated: 2026-07-24

The one formula everything rests on

Every figure on this site comes from the same piece of school physics, not from a proprietary score or a rating we invented:

kilowatt-hours = watts ÷ 1,000 × hours, and cost = kilowatt-hours × your price per kWh.

A 1,000 W appliance running 3 hours a day uses 3 kWh a day. At $0.18/kWh that is $0.54 a day. We use 30.44 days as the average month (365 ÷ 12) and 365 days as the year, so monthly and yearly figures stay consistent across every page. Enter the same numbers into any calculator here and you will get the same answer — that is the point of showing the formula rather than hiding it.

How each type of appliance is modelled

Nameplate wattage is misleading for most appliances, so we do not simply multiply it by 24. Each appliance in our dataset is modelled in one of four ways:

• Hours per day — for things you switch on and off (TV, oven, hair dryer). Energy is power × the hours it actually runs.
• Always on, with a duty cycle — for fridges, freezers and routers. A fridge's compressor does not run continuously; we apply a duty cycle, so the figure is watts × duty cycle × 24 × 365. Treating a 150 W fridge as 150 W for 24 hours would overstate its cost roughly threefold.
• Energy per cycle — for washing machines, dryers and dishwashers, where the meaningful unit is kWh per cycle × cycles per week × 52, not watts × hours.
• Hours per day, for part of the year — for air conditioners, space heaters, electric blankets and dehumidifiers. These only run during a season, not all 12 months, so multiplying daily hours by 365 would overstate the yearly cost several times over. Each of these pages carries an editable "season length" field (defaulting to a typical figure — 90 days for cooling, 120 for heating-related appliances) so the yearly total reflects a real season rather than a full year of use. We corrected this on 2026-07-22 after finding the air conditioner and space heater pages were quietly assuming 365 days of daily use — worth saying plainly here rather than pretending it was always right.

Where the prices come from

Electricity prices are residential rates for 23 countries. One definition applies to all of them: the average price per kWh with all taxes included, for the 2500–4999 kWh/year consumption band — a typical medium household. That is the number that lands on a bill, which is the only number this site promises.

Corrected 2026-07-26. Until that date the table mixed definitions: Spain carried the price excluding taxes while Germany carried it including them, and nothing here said which was which. An audit against Eurostat found six countries out by 15% or more — Spain by 29%, Sweden and Norway by 47%. All twelve countries Eurostat covers were repointed at a single source and definition, and tools/fetch-prices.mjs now refreshes them so they cannot drift again unnoticed.

Where each figure comes from:

• EU countries and Norway (12) — Eurostat dataset nrg_pc_204, electricity prices for household consumers. Published half-yearly and in arrears, so the newest official figure is always a few months old; the page prints which period it is showing.
• United States — EIA Electric Power Monthly, table 5.6.A, plus the state-by-state breakdown.
• The other 10 countries — no open data source exists, so these are checked by hand against the national regulator. Each carries the date it was last verified, and a test fails the build if any goes a year without one.
• Retailer tariffs (42) — always by hand: no open API publishes retail tariffs. They are indicative standard tariffs, not live quotes.
• Currency conversion — European Central Bank daily reference rates.

Two more things to be clear about:

• Retailer prices are indicative standard tariffs, not live quotes. Tariffs change constantly and vary by region, meter and contract. Always check your own bill.
• The US-dollar column used for cross-country comparison uses indicative exchange rates. It exists to rank countries against each other, not to price your bill.

Every price on the site is an editable starting point. The number that matters is the unit rate on your own bill, and every calculator lets you type it in.

Added 2026-07-26 — US prices state by state: the single US average hides a spread of more than four to one between states, so the state-level page lists all 50 states and the District of Columbia separately. Those figures come from the U.S. Energy Information Administration, Electric Power Monthly, table 5.6.A (average residential retail price, Form EIA-861M), with the year-earlier figure alongside. As a work of the US federal government the data is public domain; it is credited here because knowing where a number came from is the point of this page. EIA monthly figures are preliminary estimates and are averages across a whole state, so they describe the market, not your contract.

How the home estimator works

The estimator adds up a whole household instead of one appliance. Each item carries a typical annual kWh figure for normal use; you set how many you own. A few water-, cooking- and laundry-related items scale with household size using a simple factor (roughly 0.55 + 0.15 per person, capped), because a five-person home does more laundry than a one-person home.

The month-by-month chart is not a flat line divided by twelve. Heating loads are weighted towards the cold months, cooling towards the hot ones, and everything else spreads evenly as base load. Because seasons invert below the equator, the pattern flips when you select the Southern hemisphere — it is set automatically from your country and you can override it.

Added 2026-07-24 — carbon footprint estimate: the estimator also converts your electricity use into kg CO₂-equivalent per year, using each country's grid carbon intensity (grams CO₂e per kWh, lifecycle basis) — sourced from Our World in Data, compiled from Ember's Global Electricity Review 2026 (2025 generation-mix data, the most recent full year available — electricity statistics are always compiled a year in arrears). The "driving equivalent" comparison assumes an average petrol car at 180g CO₂/km, a widely used round figure rather than a per-country one. Like the cost figures, this covers electricity only.

The solar estimate, and why it is deliberately conservative

Turning on solar panels estimates annual generation as system size (kWp) × a yield figure for your country — sunnier countries get a higher yield. That generation is spread across the year (more in summer) and subtracted from each month's demand.

Crucially, we cap how much of your demand solar can cover at 55%. Without a battery you can only use solar while it is being generated, so a very large array does not take your bill to zero. Models that assume 100% self-consumption produce impressive but dishonest savings. This is a first-look what-if, not a solar quote: real self-consumption, feed-in tariffs, shading and roof orientation all matter.

What our figures deliberately exclude

Being explicit about limits matters more than looking precise. Our numbers cover electricity consumption only. They do not include:

• Standing charges and fixed supply fees, which are a real part of every bill;
• Taxes and levies, which vary by country and region;
• Tiered, time-of-use or dynamic pricing — we use a single unit rate;
• Gas for heating, cooking or hot water;
• Your specific model's efficiency, its age, or how well it is maintained.

This is why every page carries the same warning: these are well-informed estimates, not bills.

Verification, updates and corrections

Calculators are reproducible by design: the inputs and the formula are on the page, so anyone can check the arithmetic. Appliance wattages are cross-checked against energy-label data and published typical ranges rather than a single source. Price data carries the date it was compiled.

Evergreen guides are reviewed every 6–12 months and whenever the underlying facts change; each page shows its last-updated date. If something here is wrong, we would rather fix it than defend it — report it through the contact page and corrections take priority over new content.