Electricity Cost Calculator
Enter your appliance wattage (or pick a preset), daily usage, and electricity rate to see what it costs per day, per month, and per year. Adjust the duty cycle for devices like refrigerators that cycle on and off, and use the reverse-solve mode to find how long you can run an appliance within a target budget.
Formula
Worked example
A 1500 W space heater at 100% duty cycle, 4 h/day, 30 days, $0.17/kWh: effective draw = 1500 W, kWh/day = 1.5 kW x 4 h = 6 kWh, monthly kWh = 180, cost = 180 x $0.17 = $30.60.
How the calculation works
The calculator converts your appliance wattage to kilowatts, applies the duty cycle (the fraction of time the device truly draws full power), multiplies by daily hours to get kilowatt-hours per day, and then multiplies by your tariff rate. Those three numbers, kilowatts, hours, and price per kWh, are all that matter. Daily, monthly, and annual costs are derived from the same per-day figure so the results are always consistent with each other.
Power units: watts, kilowatts, BTU/h, and horsepower
Most household appliances list power in watts (W) or kilowatts (kW). HVAC equipment is often rated in BTU per hour (BTU/h), where 1 BTU/h equals about 0.293 W. Pool pumps and older motors may be rated in horsepower (hp), where 1 hp equals roughly 746 W. The calculator converts any of these to watts before computing, so you can enter the value straight from the nameplate without converting manually.
Duty cycle: why rated watts can overstate real consumption
Rated wattage is the peak draw when the heating or cooling element is fully active. Many appliances cycle: a refrigerator compressor typically runs 30-50% of the time, a thermostat-controlled space heater may run 50-70%, and an air conditioner on a mild day runs far less than on a hot one. Setting the duty cycle below 100% multiplies the rated wattage by that fraction to give an effective draw, which is far more accurate for long-running estimates. For a Kill A Watt reading or a utility-monitored figure, set duty cycle to 100% and enter the measured average wattage directly.
Reverse-solve: finding how long you can afford to run an appliance
Switch on the reverse-solve mode and enter a monthly cost target. The calculator works backwards from that budget, dividing it by the daily kWh cost at your tariff rate, to tell you the maximum hours per day that keeps you within the limit. This is useful for devices you are considering buying, to see whether the running cost fits your budget before you commit.
Typical wattage and daily cost of common appliances
| Appliance | Typical wattage | Duty cycle | Daily cost (4 h/day) |
|---|---|---|---|
| LED bulb | 8-12 W | 100% | $0.01 |
| Incandescent bulb | 40-100 W | 100% | $0.03-$0.07 |
| Ceiling fan | 15-75 W | 100% | $0.01-$0.05 |
| Laptop | 30-70 W | 100% | $0.02-$0.05 |
| Desktop PC + monitor | 150-300 W | 100% | $0.10-$0.20 |
| LED TV (55 in) | 80-150 W | 100% | $0.05-$0.10 |
| Refrigerator | 100-200 W rated | 30-50% | $0.02-$0.08 (always on) |
| Dishwasher | 1,200-1,500 W | 100% while running | $0.20-$0.26 per cycle |
| Electric dryer | 4,000-6,000 W | 100% while running | $0.68-$1.02 per cycle |
| Microwave | 900-1,200 W | 100% while running | $0.15-$0.20 per hour |
| Window AC | 900-1,800 W | 50-80% | $0.31-$0.98 (8 h) |
| Central AC | 2,000-5,000 W | 30-60% | $0.41-$2.04 (8 h) |
| Space heater (1500 W) | 1,500 W | 50-100% | $0.51-$1.02 (4 h) |
| Electric water heater | 3,000-5,500 W | 25-40% | $0.13-$0.37 (active hours) |
| EV charger (Level 2) | 6,200-7,600 W | 100% while charging | $1.06-$1.29 per hour |
Based on U.S. DOE and EIA data. Costs assume $0.17/kWh. Ranges reflect variation by model and usage pattern.
Frequently asked questions
What is a kilowatt-hour and how does it relate to watts?
A kilowatt-hour (kWh) is the energy used by a 1,000-watt device running for one hour. Watts measure instantaneous power, kWh measure the total energy consumed over time. Your utility bill charges in kWh, so multiplying effective kilowatts by hours and by your tariff gives the cost directly.
Where do I find the wattage of my appliance?
Look for a label on the back or bottom of the device showing voltage (V), current (A), or power (W). If only V and A are listed, multiply them for an approximate watt figure. For cycling or variable-load devices, a plug-in energy monitor (such as a Kill A Watt meter) gives a far more accurate average wattage than the nameplate maximum.
How do I find my electricity price per kWh?
Your rate appears on your utility bill, often labeled "energy charge" or "supply charge." The U.S. residential average is about 16-17 cents per kWh (EIA, 2024), but rates vary widely by state and country. Note that some tariffs include fixed charges and taxes, making the all-in effective cost per kWh higher than the base rate shown.
What is duty cycle and when should I adjust it?
Duty cycle is the percentage of "on" time that the heating or cooling element is actually running at full rated power. A refrigerator compressor cycles on and off, so its real consumption is 30-50% of rated watts even though it is plugged in 24 hours a day. Use 100% for devices that run continuously (LED lights, fans) and a lower percentage for cycling appliances. When in doubt, measure with a plug-in energy monitor.
How do I convert BTU/h or horsepower to watts?
Divide BTU/h by 3.412 to get watts (or multiply by 0.293). Multiply horsepower by 746 to get watts for mechanical horsepower, or by 745.7 for metric horsepower. The power-unit selector in this calculator handles the conversion automatically.
Can I use this calculator to compare two appliances?
Yes, run the calculator twice, once for each appliance, and note the annual cost for each. The difference is the yearly saving from switching. For example, comparing a 60 W incandescent bulb with a 10 W LED run 4 hours a day at $0.17/kWh shows a saving of about $2.11 per year per bulb, which adds up quickly across a whole home.