How it works
This calculator estimates how much it costs to run an electrical appliance based on its power consumption, usage pattern, and your local electricity rate. You provide four pieces of information: the device's power rating in watts, how many hours per day it operates, your electricity cost per kilowatt-hour, and the number of days you want to calculate over. The calculator then multiplies these together to give you a total energy cost.
The approach works for any appliance—refrigerators, air conditioners, laptop chargers, space heaters, washing machines—as long as you know or can estimate its wattage and daily runtime.
The formula
Total Cost = (Power in W ÷ 1000) × Hours per Day × Cost per kWh × Days
Worked example
Suppose you want to know the monthly cost of running a 1,200 W space heater for 8 hours every day, and your electricity rate is $0.14 per kWh.
Step 1: Convert watts to kilowatts.
1,200 W ÷ 1,000 = 1.2 kW
Step 2: Calculate daily energy use.
1.2 kW × 8 hours = 9.6 kWh per day
Step 3: Apply your rate.
9.6 kWh × $0.14 = $1.34 per day
Step 4: Multiply by the number of days (30 days for a month).
$1.34 × 30 = $40.32 per month
If you ran it for a full winter (90 days), the cost would be $1.34 × 90 = $120.96.
Common mistakes
Forgetting to convert watts to kilowatts: The formula requires kilowatts because electricity bills charge per kilowatt-hour. If you skip the ÷ 1,000 step, your result will be 1,000 times too high.
Using peak power instead of typical usage: Many devices don't run at full power all the time. A refrigerator rated 600 W might only draw that power intermittently; check the label for average or typical power, or use 40–60% of the peak rating as a rough estimate.
Plugging in "24 hours" for everything: Enter only the hours the device actually runs. A washing machine might run 1 hour per day, not 24. Overestimating runtime is one of the most common sources of error.
Ignoring rate variations: If your electricity rate changes seasonally or by time of day (peak vs. off-peak), calculate separate periods and add them together. This calculator assumes a constant rate.
This estimate, not professional advice: Actual electricity bills include fixed charges, taxes, demand charges, and tiered pricing structures. Use this calculator as a budgeting tool to understand appliance costs and compare energy efficiency, not as a substitute for your utility bill.