How we calculate
Every number on this site comes from one short formula and one public data source. Here is both, in full.
The formula
Electricity is billed by the kilowatt-hour, so the whole calculation is a unit conversion followed by a multiplication:
kWh = (watts ÷ 1000) × hours of use × duty cycle
cost = kWh × rate per kWh
A 1,500W space heater run for 8 hours at a 100% duty cycle uses (1500 ÷ 1000) × 8 × 1.0 = 12 kWh a day. At 18.83 cents per kWh that is $2.26 a day, about $69 a month, or roughly $825 a year if you ran it every day.
Why duty cycle matters
Nameplate wattage is what an appliance draws while actively running, not an average over time. Anything with a thermostat or compressor — refrigerators, air conditioners, water heaters, freezers — switches on and off to hold a setpoint. A refrigerator rated at 150W might only run its compressor a third of the time, so its effective draw is nearer 50W.
Ignoring this is the single most common error in appliance cost estimates, and it inflates figures for refrigeration and HVAC by a factor of two or three. We publish a duty cycle for every appliance and let you change it, because your unit, climate and settings all affect it.
Where the rates come from
Electricity rates are the average retail price of electricity to residential customers, published by the US Energy Information Administration in its Electric Power Monthly report, Table 5.6.A. We are currently using the May 2026 release, and rates refresh when the EIA publishes new figures.
The EIA is a US federal statistical agency and its data is in the public domain. We are not affiliated with, endorsed by, or connected to the EIA or any government agency.
What our figures do not include
A state average is an average. Several real costs sit outside it, and we would rather say so plainly than imply a precision we do not have:
- Your specific utility’s tariff, which can differ substantially from the state average.
- Time-of-use or tiered pricing, where the rate depends on when or how much you use.
- Fixed monthly service and delivery charges, which you pay regardless of usage.
- Local and state taxes, and any riders or surcharges on your bill.
- Demand charges, which apply to some customers on certain tariffs.
In deregulated markets — Texas most notably — retail plans vary so widely that the state average may be a poor guide to any individual bill. In every case, the most accurate thing you can do is read the per-kWh rate off your own bill and type it into the calculator.
Where the wattages come from
Typical wattages and ranges are drawn from Department of Energy appliance standards, ENERGY STAR product data, and manufacturer nameplate ratings. They are representative figures for common models, not measurements of any specific product. Real appliances vary widely, which is exactly why we show a range and make wattage an editable input rather than a fixed assumption.
Your data
The calculators run entirely in your browser. When you change a wattage or a rate, nothing is transmitted to us — the arithmetic happens locally in JavaScript. We could not see your inputs even if we wanted to.