How this calculator works
Enter an appliance's power rating in watts, how many hours a day it runs, your electricity price per kilowatt-hour, and the number of days you want the total energy figure for. The calculator returns the energy used over that period plus the cost per day, month and year at the same daily usage rate.
Watts and kilowatt-hours measure different things: watts is a rate of power draw at any instant, while a kilowatt-hour is an amount of energy used over time — a 1,000-watt appliance run for one hour uses exactly 1 kWh. Utility bills charge by the kilowatt-hour, which is why the hours-per-day figure matters as much as the wattage itself.
Typical appliance wattages (nameplate or manufacturer figures, approximate — check your own device's label for the actual rating): LED bulb 6 to 15 W, refrigerator 100 to 800 W (cycles on and off, so this is peak draw), desktop computer 200 to 500 W, microwave 600 to 1,200 W, space heater or window air conditioner 750 to 1,500 W, hair dryer 800 to 1,800 W, electric water heater 3,000 to 4,500 W.
The formulas
kWh per day = (watts ÷ 1,000) × hours per dayEnergy used = kWh per day × number of daysCost per day = kWh per day × price per kWh; cost per month = cost per day × 30; cost per year = cost per day × 365Month and year figures use flat 30-day and 365-day conventions for a simple, consistent projection from the daily rate — actual monthly bills vary slightly with the calendar and with how much the appliance is actually used that month.
Worked example: a 1,500 W space heater
- Space heater rated at 1,500 W, run 3 hours a day, electricity priced at $0.15 per kWh.
- Energy per day: (1,500 ÷ 1,000) × 3 = 4.5 kWh.
- Cost per day: 4.5 × $0.15 = $0.675.
- Cost per month: 0.675 × 30 ≈ $20.25. Cost per year: 0.675 × 365 ≈ $246.37.
Frequently asked questions
Where do I find an appliance's wattage?
Check the nameplate or label on the device itself, usually near the power cord or on the bottom or back panel, which lists watts (or volts and amps, which multiply to watts). The wattage ranges in this article are typical manufacturer figures only, not a substitute for your specific appliance's rating.
Why does my refrigerator's actual electricity use seem lower than its wattage suggests?
Refrigerators, air conditioners and other thermostat-controlled appliances cycle on and off rather than running continuously at their rated wattage. The nameplate lists peak draw when the compressor is running, so actual daily energy use is lower than wattage times 24 hours.
How do I find my price per kWh?
It's on your electricity bill, usually listed per kWh alongside any fixed monthly charges, delivery fees or taxes that this calculator does not include. Rates vary widely by location and by time of use on some plans, so use the rate that applies to when the appliance typically runs.
Does this calculator include fixed charges or taxes on my bill?
No — it only projects the variable energy cost from wattage, hours of use and your per-kWh rate. Most utility bills add a fixed monthly service charge and sometimes taxes or delivery fees on top of the energy cost calculated here.
How can I reduce an appliance's electricity cost?
Reduce hours of use, replace it with a lower-wattage model (LED bulbs versus incandescent is the clearest example), or shift use to off-peak hours if your utility charges variable rates by time of day.