An electricity calculator converts an appliance’s wattage and running time into kilowatt-hours and multiplies by your rate to show what it costs to run. Formula: kWh = (watts × hours × days) / 1000. Cost = kWh × rate kwh. A 100-watt device used 5 hours a day for 30 days uses 15 kWh, or $2.25 at $0.15 per kWh.
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How to use the electricity calculator
- Enter the appliance’s power in watts.
- Enter hours of use per day and the number of days.
- Enter your electricity rate per kWh.
- Read the kWh used and the cost.
What formula does the electricity calculator use?
Utilities bill you for energy in kilowatt-hours (kWh). One kWh is 1,000 watts used for one hour. The electricity calculator converts an appliance’s power and running time into kWh, then multiplies by your rate:
- kWh = watts × hours per day × days ÷ 1,000
- Cost = kWh × price per kWh
Electricity calculator worked example
A 100-watt device runs 5 hours a day for 30 days: 100 × 5 × 30 ÷ 1,000 = 15 kWh. At $0.15 per kWh, that costs $2.25 a month. At the July 2026 U.S. residential average of 18.31 cents per kWh reported by the U.S. Energy Information Administration, the same 15 kWh costs about $2.75.
Example appliance costs
The table uses example wattages and 30 days at the national average of 18.31¢/kWh. Check your appliance’s label or manual for its actual wattage.
| Appliance | Power | Hours/day | kWh/month | Cost/month |
|---|---|---|---|---|
| LED bulb | 10 W | 5 h | 1.5 | $0.27 |
| Laptop | 50 W | 8 h | 12.0 | $2.20 |
| Ceiling fan | 60 W | 8 h | 14.4 | $2.64 |
| TV | 100 W | 5 h | 15.0 | $2.75 |
| Refrigerator (average draw) | 150 W | 24 h | 108.0 | $19.77 |
| Space heater | 1500 W | 4 h | 180.0 | $32.96 |
| Window AC | 900 W | 8 h | 216.0 | $39.55 |
Heating and cooling devices draw far more power than electronics, so a few hours of a space heater can cost more than running a laptop all month. A refrigerator’s compressor cycles on and off, so its average draw is much lower than its rated wattage.
Finding your electricity rate
Divide your bill’s total by the kWh used to find your effective rate, including delivery charges and taxes. Prices vary widely by state and season. EIA data show the U.S. residential average rose from 17.45¢ per kWh in July 2025 to 18.31¢ in July 2026. Some utilities charge time-of-use rates, which are higher in late afternoon and evening; if you have one, enter the rate for the hours the appliance actually runs.
Finding watts
- Look for a wattage on the label or nameplate.
- If only amps and volts are listed, watts ≈ volts × amps; a 120-volt device drawing 5 amps uses about 600 watts.
- A plug-in energy monitor measures real use, which is often lower than the rated maximum.
- Standby power from devices left plugged in adds up; a 5-watt standby load all month is 3.6 kWh.
Ways to cut the bill
Swap remaining incandescent bulbs for LEDs, which use a fraction of the power for the same light. Use smart power strips to cut standby loads, wash laundry in cold water and run full dishwasher loads. For heating and cooling, a programmable thermostat and sealing air leaks usually save the most. Use the electricity calculator to compare an old appliance with an efficient replacement: the difference in monthly cost, times the months you will own it, shows whether the upgrade pays off.
Larger loads
The same formula works for big loads. Charging an electric vehicle that uses 30 kWh a week costs about $5.49 a week at 18.31¢. A 1,500-watt space heater running 8 hours a day for a 30-day month uses 360 kWh, about $66. Enter these values into the electricity calculator to see your own numbers.
Estimating a whole bill
To estimate a full monthly bill, list your main appliances, run each through the electricity calculator and add the results. Heating, cooling, water heating and laundry usually make up the largest share, so focus on those first. Compare the total with the kWh shown on a recent bill; if your estimate is much lower, look for something you missed, such as a well pump, a pool pump, a second refrigerator or electric baseboard heat. Seasonal changes matter too: an air conditioner that runs 8 hours a day in July may barely run in October, so estimate each season separately.
Related converters and calculators
Related calculators:
- Ohm’s law calculator: volts and amps
- Budget calculator: budget
- Fuel cost calculator: fuel
- Unit converter: units
Thinking about solar? The solar panel payback calculator estimates how long savings take to cover the cost.
Sources
U.S. EIA Electric Power Monthly, Table 5.6.A: Average price of electricity by state.
Questions about this calculator
How do I calculate kWh?
Watts × hours ÷ 1,000.
What is the average U.S. electricity rate?
18.31¢ per kWh for residential customers in July 2026, according to the EIA.
How much does a 1,500 W heater cost per hour?
1.5 kWh, about 27¢ at 18.31¢ per kWh.
How do I convert amps to watts?
Multiply volts by amps.
Why is my bill higher than the estimate?
Rates may include fees, and actual wattage and hours vary.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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