The Department of Energy's formula is simple: kWh = watts × hours ÷ 1,000, and cost = kWh × your price per kWh. A 1,500-watt space heater run 4 hours a day for 30 days uses 180 kWh; at 18¢ per kWh that is $32.40 a month, or 27¢ for each hour it runs.
The default price of $0.18/kWh is close to the U.S. average residential price of 18.16¢ that the Energy Information Administration reported for August 2026. Prices vary widely by state and plan, so use the rate from your own bill for a real estimate.
How to use the electricity cost calculator
- Find the wattage on the appliance label or measure it with a plug-in meter.
- Estimate the hours it runs each day and the days per month.
- Enter your price per kWh (total bill ÷ kWh used gives an all-in average).
- Read the monthly cost, kWh, and the daily, yearly and hourly cost.
Formula
Appliances that cycle on and off (refrigerators, heaters on a thermostat, air conditioners) don't draw their rated watts all the time, so the label wattage overstates their use. A plug-in meter gives the real number.
Worked example
1,500 W space heater, 4 hours a day, 30 days, $0.18/kWh.
- 1,500 × 4 ÷ 1,000 = 6 kWh a day.
- 6 × 30 = 180 kWh a month.
- 180 × $0.18 = $32.40 a month ($1.08 a day; $0.27 per hour).
Cost per hour at 18¢/kWh
| Wattage | Example | Per hour | 4 h/day for 30 days |
|---|---|---|---|
| 10 W | LED bulb | $0.002 | $0.22 |
| 100 W | TV or laptop setup | $0.018 | $2.16 |
| 500 W | Gaming PC under load | $0.09 | $10.80 |
| 1,000 W | Microwave | $0.18 | $21.60 |
| 1,500 W | Space heater | $0.27 | $32.40 |
The example wattages are illustrative; check your own device. Utilities may add fixed charges, tiered rates or time-of-use pricing, so your bill's per-kWh price can differ from the marginal cost. For heating and cooling capacity, the room air conditioner btu calculator sizes a unit by room; 1 kWh = 3,412 BTU.
Finding an appliance's wattage
Look for a label on the back or bottom of the appliance, or on its power adapter. If it lists only amps and volts, multiply them: a device rated 2 A at 120 V can draw up to 240 W. The label shows the maximum; many devices use less most of the time. An inexpensive plug-in watt meter measures the actual kWh over a day or a week, which is the most accurate input for this calculator: enter the measured kWh per day ÷ hours used × 1,000 as the wattage, or simply multiply the measured kWh by your rate.
Comparing an upgrade
To see whether a more efficient appliance pays for itself, run the calculator twice with the old and new wattage and the same hours, then divide the price difference by the yearly saving. Example: replacing ten 60 W incandescent bulbs with 9 W LEDs used 3 hours a day saves (600 − 90) W × 3 h × 365 ÷ 1,000 = 558 kWh a year, about $100 at 18¢/kWh.
Caveats and assumptions
- Rated watts overstate the use of appliances that cycle.
- Ignores fixed charges, tiers and time-of-use pricing.
- Default rate is a national average, not your rate.
Frequently asked questions
How do I calculate the cost to run an appliance?
Watts × hours ÷ 1,000 gives kWh; multiply by your price per kWh. 1,500 W for 4 hours at 18¢ = $1.08.
How much does it cost to run a 1,500-watt heater?
About 27¢ per hour at 18¢ per kWh, or $32.40 a month at 4 hours a day.
How do I convert watts to kWh?
Multiply watts by hours of use and divide by 1,000. 100 W for 10 hours = 1 kWh.
What is the average price of electricity?
EIA reported an average U.S. residential price of 18.16¢ per kWh for August 2026. Your rate depends on your state and plan.
How do I find my price per kWh?
Your bill lists the rate; for an all-in average, divide the total bill by the kWh used that month.
Sources
- U.S. DOE — "Watt Does It Cost to Use It?" (PDF) — watts ÷ 1,000 = kW; kWh = kW × hours; cost = $/kWh × kWh
- EIA Electric Power Monthly, Table 5.6.A — Average price by state — U.S. residential average 18.16¢/kWh, August 2026
- NIST SP 811 Appendix B.9 — Conversion factors — kWh = 3.6 MJ; Btu conversions
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.