The formula: watts, hours and your rate
Electricity is billed by the kilowatt-hour, the energy used by a 1,000-watt load running for one hour. Three numbers give you the cost of anything that plugs in: the power draw in watts, the hours you run it, and the price you pay per kWh. Energy per day equals watts times hours divided by 1,000. Cost equals that figure times your rate. Nothing else is involved, which is why a quick estimate here is usually within a few percent of what shows up on the bill.
Worked example with the defaults: a 1,500-watt space heater running 4 hours a day uses 1,500 x 4 / 1,000 = 6 kWh a day. At 17 cents per kWh that is 6 x 0.17 = $1.02 a day, $30.60 over a 30-day month, and $372.30 if you ran it that way all year. Cut the run time to 2 hours a day and the monthly cost falls to $15.30. Nothing about the heater changes; only the hours do.
Finding your real rate
The number to use is not the supply rate printed in an advertisement. Take your last bill, divide the total dollars charged by the kilowatt-hours used, and you get the blended rate that includes generation, transmission, distribution, and fixed customer charges spread over your usage. That blended figure is often 3 to 6 cents higher than the headline supply rate. The national residential average is roughly 16 to 17 cents per kWh, but state averages run from about 11 cents in Idaho and Utah to more than 40 cents in Hawaii.
Many utilities use tiered or time-of-use pricing. Tiered plans charge a low rate for a baseline block of kWh and a higher rate above it, so an extra appliance is billed at the top tier, not the average. Time-of-use plans charge two or three times as much during a peak window, often 4 p.m. to 9 p.m. on weekdays. If you are on either plan, enter the marginal rate that the extra usage would actually fall into, not the average, and consider shifting the dryer or the EV charger to off-peak hours.
What actually drives a big bill
Heating, cooling and hot water dominate US household electricity use; lighting and electronics are rounding errors by comparison. A single 1,500-watt space heater costs more to run than every LED bulb in the house combined. A central air conditioner drawing 3,500 watts for 6 hours a day costs about $107 a month at 17 cents. An electric water heater at 4,000 watts for 3 hours a day runs about $61 a month. A 9-watt LED bulb on 5 hours a day costs about 23 cents a month.
Electric vehicles shift a fuel expense onto the electric bill. A Level 2 charger pulling 7,200 watts adds roughly 7.2 kWh per hour of charging; 1,000 miles of driving at 30 kWh per 100 miles is 300 kWh, or about $51 a month at 17 cents. That is still far less than gasoline for the same mileage. Phantom loads from always-on devices are real but small: a cable box, game console and a few chargers together typically total 30 to 60 kWh a month, $5 to $10.