The estimate converts rated watts into kilowatt-hours. Use measured power or average operating wattage for devices that cycle on and off.
Find the right wattage
Look for a nameplate, manual, power adapter, or manufacturer specification. A plug-in power meter can provide a better average for devices whose consumption changes during use.
A rated maximum can overstate energy use when equipment cycles. Refrigerators, air conditioners, heat pumps, and computers often draw different power at different times.
Read the electricity price from your bill
A simple bill estimate works best with an all-in variable price per kWh. Divide relevant usage-based charges by billed kWh when the utility does not show one clear figure.
Fixed customer charges do not change when one appliance uses more energy, so this calculator does not add them. Time-of-use, tiered, or demand pricing requires a more detailed schedule.
Test practical savings
Change hours per day, device count, or wattage to compare alternatives. Reducing operating time often has a direct proportional effect in this model.
For heating or cooling, a lower wattage device may need to run longer. Compare delivered comfort or useful output, not only the nameplate rating.
The formula used
Monthly cost = watts ÷ 1,000 × hours/day × days × devices × price/kWhOne kilowatt-hour equals using 1,000 watts for one hour. The entered electricity price should use the same currency as the desired result.
Example: a 1,500-watt heater
A 1,500-watt heater used 6 hours a day for 30 days consumes an estimated 270 kWh. At $0.18 per kWh, the energy portion of the monthly cost is about $48.60.
Sources and further reading
We use primary educational sources to check terminology and explain how the calculation fits into real financial decisions.
U.S. Department of Energy — Estimating appliance energy use↗U.S. Energy Information Administration — Electricity prices↗