How we calculate

Every figure on this site is produced by one piece of code from one set of inputs. This page says what that code does, what it assumes, and where it stops being reliable.

Reviewed 20 August 2026  ·  Next review 20 February 2027  ·  Report an error

The basic identity

Electricity is billed by the kilowatt-hour. Every calculation here reduces to the same two steps: work out how many kilowatt-hours a day the appliance uses, then multiply by the price you pay per kilowatt-hour. Everything else is about getting the first step right.

Why one formula is not enough

Power in watts multiplied by hours run is correct for a space heater and wrong for almost everything else. A refrigerator is plugged in for 24 hours but its compressor runs for about a third of them. A dishwasher does not run for an hour every day — it runs a cycle a few times a week. A heat pump moves heat rather than making it, so it delivers three to four units of warmth per unit of electricity. And an electric car's charging cost is set by how far you drove, not by how many watts the charger is rated at.

Using one formula for all of those produces confident numbers that are quietly wrong, and savings advice that does not survive contact with the physics — "run your charger an hour less" is not a saving, it is 30 fewer miles of driving. So each appliance declares which model it belongs to:

The month, the year and the season

A month is 30.44 days — the average calendar month — everywhere on the site, so a monthly figure in a FAQ answer and the same figure in the calculator above it cannot drift apart. A year is 365 days of the usage shown.

That annual figure answers "if I ran it like this every day for a year". For appliances with a real season — heaters, air conditioning, holiday lights — the page also gives the season's cost, because an annual total quietly implies year-round use and a space heater is not run in July.

The off-peak assumption on the EV page

The EV charger's saving scenario asks what shifting all charging to an off-peak window would do. If you have entered your own off-peak price, that is the price it uses. If you have not, it assumes an off-peak rate of 65% of your standard rate and says so in the sentence, because a saving figure with a hidden tariff behind it is not checkable.

That percentage is a planning assumption, not a measured tariff: real time-of-use plans vary widely, and some have no cheap window at all. Enter your own off-peak price and the scenario uses it instead — that is the number worth trusting.

Where the prices come from

U.S. prices are average residential revenue per kilowatt-hour from the U.S. Energy Information Administration's Electric Power Monthly, Table 5.6.B, June 2026 year to date. They are generated into the site at build time from a single source file, with the period and retrieval date attached, and the build refuses to publish a table that fails validation. The visitor's browser never fetches a price: there is no live rate API to go down, rate-limit or observe you.

All 52 U.S. rows imported from the published EIA residential year-to-date table.

International prices are rounded typical household tariffs for each market, in that market's own currency unless the row says US$ equivalent. These are planning benchmarks with no single primary source; enter the per-kWh price from your own bill for an accurate figure.

Where the wattages come from

Typical running power for each appliance is drawn from U.S. Department of Energy and ENERGY STAR appliance guidance and published manufacturer specifications, expressed as a typical figure and a usual range. They are representative, not a measurement of your unit. The nameplate on your appliance, or an hour with a plug-in energy meter, beats any typical figure we can publish.

What these numbers are not

• They are planning estimates, not utility quotes, engineering calculations or a guarantee of savings.
• They do not model your tariff's fixed charges, tiers, time-of-use windows, demand charges, taxes or fuel adjustments — only the per-kWh price you enter.
• They assume the appliance behaves like a typical one of its kind. An old, badly sited or faulty unit can use far more.
• Heating and cooling figures depend on climate, insulation and how you actually use the equipment. Treat them as a scenario, not a forecast.

How we keep the numbers consistent

The calculator, the worked examples, the FAQ answers, the structured data and the page descriptions are all generated from the same calculation for the same appliance. A build-time check compares every published figure against that calculation and fails the build if any of them disagree, which is how a page showing one monthly cost in its widget and a different one in its FAQ stops being possible.