The formula
annual kWh = (watts x dutyCycle x hoursPerDay x daysPerYear / 1000) + (standbyWatts x (24 - hoursPerDay) x daysPerYear / 1000)Almost every published ‘cost to run' figure multiplies nameplate watts by hours. That is the number on the sticker, not the number on the meter. Anything with a thermostat or a compressor cycles off once it reaches setpoint, so its average draw over an hour is a fraction of its rating. Ignoring that overstates a space heater by roughly 2x and a refrigerator by roughly 3x.
Duty cycle
The fraction of its in-service hours that an appliance is actually drawing its rated power. A refrigerator’s compressor runs roughly a third of the time, so its duty cycle is about 0.32 and its average draw is 48 W against a 150 W rating. A kettle draws full power until it boils and then stops, so its duty cycle is 1.0 and nameplate arithmetic is correct for it.
Standby
Counted separately, over the hours the appliance is not in service. At 18.34¢ per kWh, one watt drawn continuously for a year costs $1.61.
Where the inputs come from
Nameplate and standby figures are typical published manufacturer ratings for mainstream current models. Duty cycles are engineering values for the stated operating assumption. Every figure is a typical value for a typical unit, not a measurement of any specific product; ranges are given where the spread across models is wide enough to change the answer.
- FTC EnergyGuide label estimated yearly electricity use (16 CFR Part 305)
- U.S. DOE appliance efficiency standards test procedures (10 CFR Part 430)
- ENERGY STAR product specifications and qualified product lists
- U.S. EIA 2020 Residential Energy Consumption Survey, end-use tables
How the appliance figures are calibrated
Where an appliance carries a regulated EnergyGuide or ENERGY STAR annual figure, the model’s default inputs are set so the computed annual kWh lands within that published figure’s range. Those appliances carry “calibratedTo”.
Concretely: US appliance efficiency regulation requires many products to carry a tested annual consumption figure. Where one exists, the default inputs on this site are set so the computed annual total lands inside that published range. The build fails if it does not — the check runs on every build and is listed below.
| Appliance | Computed | Published range | Basis |
|---|---|---|---|
| Refrigerator | 420 kWh | 350–500 kWh | FTC EnergyGuide typical range |
| Chest freezer | 342 kWh | 200–400 kWh | ENERGY STAR criteria by size |
| Dishwasher | 244 kWh | 200–270 kWh | ENERGY STAR criteria |
| Clothes washer | 97 kWh | 85–150 kWh | ENERGY STAR, excluding water heating |
| Electric dryer | 682 kWh | 600–900 kWh | FTC EnergyGuide typical range |
| Electric water heater | 2,720 kWh | 2,300–3,100 kWh | EIA RECS conditional mean, 2,706 kWh |
| Central air conditioning | 2,295 kWh | 1,900–2,700 kWh | EIA RECS conditional mean, 2,318 kWh |
Where a duty cycle comes from
This is the input that does the most work, so it deserves the most explanation. A duty cycle here is an engineering value for the operating assumption stated on the appliance’s own page — not a measurement of a specific unit, and not a number chosen to make an answer come out well.
Three things constrain it:
- Physics. A thermostatic appliance runs long enough to replace the heat it lost or gained, which is set by the temperature difference it is fighting and how well insulated it is. A freezer in a hot garage cycles more than the same freezer indoors, and the model says so.
- The published annual figure, where one exists. Regulation requires many appliances to carry a tested annual consumption number. Where there is one, the duty cycle is set so the computed total lands inside it. That is not curve-fitting — it is the only external check available, and it is the difference between a model and a guess.
- Consistency across the set. Appliances that work the same way get comparable treatment. Every compressor-driven cold appliance sits in a similar band, and where one does not, the page says why.
Appliances with no thermostat get a duty cycle of 1.0, because there is nothing to cycle. For those, nameplate arithmetic is correct, and this site says so rather than manufacturing a difference — 60 of the 120 appliances here fall into that group.
Standby, and why it is counted separately
Duty cycle covers the hours an appliance is in service. Standby covers the hours it is not, at a much lower draw, and the two are added rather than blended. Keeping them apart matters because they respond to different things: standby is a property of the device’s design and is unaffected by how much you use it, while running energy scales directly with use.
It also makes an otherwise odd result legible. An appliance used fifteen minutes a day can still carry a meaningful annual figure, because the other twenty-three and three-quarter hours are not free. At 18.34¢ per kWh, one watt drawn continuously for a year costs $1.61 — so a three-watt clock display is $4.82 a year whether or not the appliance is ever switched on.
The bill breakdown
End-use figures come from the EIA’s 2020 Residential Energy Consumption Survey, table CE4.6. Those averages are conditional: the space-heating figure of 2,484 kWh is the average among households that heat with electricity, not among all households. That is why the end-use columns in the source table do not sum to the total column, and it is why applying them only to the end uses a home actually has produces a breakdown that reconciles.
| End use | Average, among homes that use electricity for it |
|---|---|
| Space heating | 2,484 kWh |
| Water heating | 2,706 kWh |
| Air conditioning | 2,318 kWh |
| Refrigeration | 839 kWh |
| All other uses | 5,108 kWh |
| Total, all households | 10,566 kWh |
A worked example, end to end
Taking the refrigerator, because it is the appliance the standard method gets most wrong.
- The plate says 150 W. That is the compressor's draw while it is running, and it is the figure a breaker must be sized for.
- The compressor runs roughly a third of the time in a normal kitchen — it cycles to hold the cabinet temperature. So the average draw across the day is 150 × 0.32 = 48 W.
- It is in service 24 hours a day, 365 days a year, because a fridge is never off. 48 × 24 × 365 ÷ 1,000 = 420 kWh a year.
- At 18.34¢ that is $77 a year.
- The standard method — 150 × 24 × 365 ÷ 1,000 — gives 1,314 kWh, or $241. That is 3.1× the real figure.
The check that settles which of the two is right is external: FTC EnergyGuide labels put a full-size refrigerator in the region of 350–500 kWh a year. 420 kWh sits inside that range. 1,314 kWh is nearly three times its upper bound, which is not a modelling disagreement — it is an answer that could not be produced by any refrigerator sold.
What this method cannot tell you
These are estimates for typical equipment under stated assumptions. They are not a measurement of your home, and there are several things they genuinely cannot capture:
- Your specific unit. A fifteen-year-old refrigerator with a tired seal can use double what a current one does. The model describes a mainstream current model in ordinary condition.
- Your climate. Heating and cooling vary by a factor of four across US climate zones. The defaults describe a mixed climate; the sliders on each page are there because of this.
- Your tariff. Statewide averages include fixed monthly charges spread across the kilowatt-hours sold, and they cannot see time-of-use pricing at all. Divide your bill total by its kilowatt-hours to get the number that actually applies to you.
- Behaviour. Two identical houses with identical equipment routinely differ by half, and nothing in a model can see that.
Corrections
If something here is wrong, write to contact@wattdraw.com with the figure and the source you think is better. Corrections are made in the data files the site is generated from, so a fix propagates to every page that quotes the number.
Source: U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A, June 2026. U.S. Government work, public domain.
Source: U.S. EIA, 2020 Residential Energy Consumption Survey (RECS), Table CE4.6. U.S. Government work, public domain.