Electrical Power Consumption: How to Calculate What a Device Costs to Run
A device's monthly running cost is its wattage × hours per day × 30, divided by 1,000 to get kilowatt-hours, times your rate per kWh. A 1,500 W space heater run 4 hours a day at $0.18/kWh costs $32.40 a month. Run that calculation for each appliance and the bill stops being a mystery: a few devices usually dominate, and some of the surprising ones are small loads that never switch off. This guide shows the formula, a worked example and the inputs that most often go wrong.
Watts Versus Kilowatt-Hours
A watt is the rate at which a device draws power at an instant; a kilowatt-hour is the energy used when a 1,000 W draw continues for an hour, and it is the unit your utility bills. Two devices with the same wattage can cost very different amounts depending on run time: a 1,500 W hair dryer used five minutes a day is cheap, while a 50 W aquarium pump running around the clock quietly adds up.
How to Calculate Monthly Running Cost
Monthly cost = Power (W) × Hours per day × 30 ÷ 1,000 × Rate ($/kWh)
Power × hours gives watt-hours per day; × 30 makes it a month; ÷ 1,000 converts to kWh; × rate converts to dollars.
Worked example. A space heater in a home office:
- Power: 1,500 W
- Daily use: 4 hours
- Electricity rate: $0.18 per kWh (close to the 2026 U.S. residential average of about 18 cents per kWh reported by the EIA)
2. Monthly energy: 6,000 × 30 = 180,000 Wh = 180 kWh
3. Cost: 180 × $0.18 = $32.40 per month
The Electrical Power Consumption Calculator takes wattage, hours per day and rate and returns this monthly figure (it assumes a 30-day month). For an annual figure that accounts for days per week and several identical units, multiply the daily cost by days per week × 52 and by quantity.
Putting the Numbers to Work
Rank your loads. Heating, cooling, water heaters and electric dryers usually top the list, but a second refrigerator running 24 hours a day often beats an occasional high-wattage device.
Compare replacements. A 60 W incandescent bulb used 4 hours a day costs 60 × 4 × 30 ÷ 1,000 × $0.18 = $1.30 a month; a 9 W LED giving similar light costs $0.19. Per bulb the gap is small; across a house with 30 bulbs it is about $33 a month.
Check the rate on your bill. The rate is half the equation. On a time-of-use plan, shifting a dryer or dishwasher to off-peak hours changes the cost without changing the energy.
Convert amps and volts. Some nameplates give amps and volts rather than watts. Multiply them, or use the Electrical Power Calculator, before entering the wattage.
Common Mistakes
Confusing watts with watt-hours. A 1,500 W device only uses 1,500 Wh if it runs for exactly one hour.
Using nameplate wattage for cycling loads. A refrigerator's compressor runs part of the time and a thermostat-controlled heater idles once the room is warm. Nameplate wattage × 24 hours overstates these badly; estimate actual run time, or use a plug-in energy meter.
Ignoring standby power. Devices in standby draw a few watts all day. Lawrence Berkeley National Laboratory and the U.S. Department of Energy put standby at roughly 5–10% of a typical U.S. home's electricity use, so 24 hours a day is a large multiplier even for small draws.
Using a national average rate. Prices differ by region, season and usage tier. Use the rate printed on your own bill.
Related guide: Electrical Power: How to Calculate Watts from Voltage and Current.