The formula behind the numbers
Utilities bill energy, not power. Power, in watts, is how fast an appliance draws electricity at a given moment; energy, in kilowatt-hours, is that draw kept up over time. The conversion used here comes from Virginia Cooperative Extension's fact sheet on estimating appliance energy use, which was developed as part of the NASULGC/DOE Building Science Community of Practice: wattage times hours used per day, divided by 1,000, gives daily kilowatt-hour consumption.
From there the fact sheet multiplies by the number of days the appliance is used for a period total, and by the utility's rate per kWh for the cost. The calculator above does exactly that, keeping full precision and rounding only what it displays.
The fact sheet's own worked example is a useful check. A 200 W window fan run 4 hours a day for 120 days uses (200 × 4 × 120) ÷ 1,000 = 96 kWh. At the 8.5 cents per kWh rate it uses for illustration, that comes to $8.16 a year. Enter 200 W, 4 hours, a custom rate of 8.5 cents and 120 days, and the tool returns the same 96 kWh and $8.16. The site's test suite checks both that example and the fact sheet's computer-plus-monitor example before the page is built.
Three inputs most calculators leave out
No wattage on the label. The fact sheet says you can estimate it from the current draw in amperes multiplied by the voltage. Most US appliances use 120 volts; larger ones such as clothes dryers and electric cooktops use 240. Switch the power field to Amps and pick the voltage, and the calculator does the multiplication. The fact sheet adds that the amperes are sometimes stamped on the unit in place of the wattage, so check the plate for an amp figure before reaching for a meter.
Appliances that cycle. A refrigerator is plugged in all day but its compressor does not run all day. The fact sheet's rule of thumb is to divide the time it is plugged in by three to estimate the hours at full wattage. The "share of time at full power" field handles that: 24 hours at 33% is the same as 8 hours at full draw. The same field works for anything with a thermostat, such as a space heater that clicks on and off.
Power drawn while off. The fact sheet calls these phantom loads and notes they occur in most appliances that use electricity, from televisions to kitchen appliances. Enter the standby watts, and the calculator charges them for every hour of the day the appliance is switched off. A TV that draws 120 W for 4 hours and 5 W on standby for the other 20 hours uses 0.58 kWh a day, and about 17% of that comes from standby. The fact sheet says most phantom loads add only a few watt-hours, and that they can be avoided by unplugging the appliance or by using the switch on a power strip to cut all power to it.
Which rate to use
The most accurate rate is the one on your own bill. Choose "My own rate" and enter it in cents per kWh. If your bill shows separate supply and delivery charges per kWh, add them together.
If you do not have a bill to hand, pick your state. The list uses Table 5.6.A of the US Energy Information Administration's Electric Power Monthly, the average residential price for July 2026, which is the latest month in the table at the time of writing. EIA marks these values as preliminary estimates. An average price is total revenue divided by total kWh sold, so it spreads delivery and other per-kWh charges into one number. It does not reflect fixed monthly customer charges, tiered rates or time-of-use pricing, which is why your own bill can come out higher or lower.
The spread between states is large. July 2026 residential prices ran from 12.72 cents per kWh in Louisiana to 48.00 cents in Hawaii. The same 1,500 W heater used 4 hours a day for 30 days costs about $22.90 at Louisiana's average and $86.40 at Hawaii's.
How rates moved over the past year
The same EIA table prints July 2025 next to July 2026, so the tool can also show how much a fixed habit would have changed in cost. The US average residential price rose from 17.45 to 18.31 cents per kWh. Prices were higher than a year earlier in 45 of the 50 states and the District of Columbia.
The largest rises were in Hawaii, up 8.64 cents (22.0%), Maine, up 4.43 cents (15.8%), New Hampshire, up 3.81 cents (16.7%), and New York, up 3.68 cents (14.0%). Connecticut went the other way, down 3.44 cents (12.5%). Five other states also dipped slightly: Louisiana (down 0.40 cents), Utah (down 0.33), Wyoming (down 0.28), New Jersey (down 0.13) and Florida (down 0.08).
Monthly figures move with the season, so treat a single month as a snapshot rather than the price you will pay all year. When EIA publishes a newer month, the state list on this page should be refreshed from the same table.
Getting a better estimate for one appliance
The label wattage is a ceiling, not a typical draw. The fact sheet notes that the wattage listed is the power drawn by the appliance and that, because many appliances have a range of settings, the actual amount used depends on the setting at any one time. A heater on its low setting or a dimmed TV uses less than the plate says. The fact sheet's own list shows how wide those ranges can be: a clothes dryer at 1,800 to 5,000 W, and a dishwasher at 1,200 to 2,400 W, where using the drying feature greatly increases energy consumption.
For a precise number, measure. The fact sheet suggests a clamp-on ammeter, an electrician's tool that clamps around one of the two wires, read while the appliance is running. It also warns that a motor draws about three times more current while starting than when running smoothly, so read it once it has settled.
For whole-home cost, run each major appliance separately and add the monthly totals, then compare with the kWh on your bill. If your total is far below the bill, the gap usually sits in the big heating, cooling and water-heating loads, which have the highest wattages. The fact sheet lists a 40-gallon water heater at 4,500 to 5,500 W, so one hour of its run time uses as much energy as a 50 W laptop running nonstop for 90 to 110 hours. Those are the loads worth measuring first. If you are also planning a room upgrade, the square footage calculator handles the floor-area side.