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 $0.17/kWh that is $0.51 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 typical residential rates for 23 countries, compiled from public national regulator and statistics-office figures (for example Eurostat, the US EIA, Ofgem and CNMC), plus indicative standard-tariff prices for the main retailers in each market. Two 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.

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.