Understanding your appliance's energy use
The following table gives typical wattages for common household appliances to sanity-check your inputs; always use the specific appliance's own rated wattage where available.
| Appliance | Typical running power |
|---|---|
| LED light bulb | 5 – 15 W |
| Laptop computer | 30 – 65 W |
| Refrigerator (average, cycling) | 100 – 200 W |
| Space heater | 1,000 – 1,500 W |
| Electric oven (baking) | 2,000 – 3,000 W |
- Rated wattage on an appliance label is often a maximum; average running power for cycling appliances (fridges, air conditioners, heaters with thermostats) is typically lower than the rated peak.
- This calculator uses a flat electricity rate; it does not model tiered pricing, time-of-use rates, or standing/service charges that appear on many utility bills.
What does this electricity cost calculator do?
Electricity is billed by the kilowatt-hour (kWh) — the energy used by a 1,000-watt device running for one hour. This calculator converts an appliance's power rating (in watts) and its daily hours of use into kWh, then multiplies by the electricity rate to estimate cost. Monthly and yearly figures use the average number of days per month (30.44) and days per year (365.25) to account for leap years and varying month lengths.
The wattage of an appliance is usually printed on a label, in the manual, or on the manufacturer's spec sheet — use the running (not peak/startup) wattage for the most accurate estimate. Devices with variable loads, such as refrigerators or air conditioners that cycle on and off, are better estimated using their average duty-cycle wattage rather than their rated maximum.
How to use this electricity cost calculator
- Enter the appliance's power draw in watts, found on its label or spec sheet.
- Enter the average number of hours per day the appliance runs.
- Enter your electricity rate per kWh, found on your utility bill.
- Read the daily and yearly energy use in kWh, plus the estimated daily, monthly and yearly cost.
The formula behind electricity cost
Energy in kilowatt-hours equals power in watts multiplied by hours of use, divided by 1,000 (to convert watts to kilowatts). Cost equals energy (kWh) multiplied by the electricity rate per kWh. Monthly cost scales daily cost by the average days per month (365.25 ÷ 12 ≈ 30.44); yearly cost scales it by 365.25 days to average out leap years.
Worked example: a 1,500 W appliance run 4 hours per day at $0.25/kWh uses 1,500 × 4 ÷ 1,000 = 6 kWh per day, costing 6 × $0.25 = $1.50 per day. Over a month that is roughly 6 × 30.44 × $0.25 ≈ $45.66, and over a year roughly $547.88.
Common mistakes
- Using an appliance's peak/startup wattage instead of its average running wattage.
- Entering the electricity rate in cents when the calculator expects dollars (or vice versa), which changes the result by 100×.
- Forgetting that cycling appliances (fridges, AC units) run intermittently, so 24 hours of 'plugged in' time overstates actual power-on hours.
- Ignoring tiered or time-of-use electricity pricing, which this flat-rate estimate does not capture.
よくある質問
How much does it cost to run a 1500W appliance for 4 hours a day?
At $0.25 per kWh, a 1,500 W appliance run 4 hours daily uses 6 kWh per day, costing $1.50 daily, about $45.66 per month, and about $547.88 per year.
How do I convert watts to kilowatt-hours?
Kilowatt-hours equal watts multiplied by the number of hours used, divided by 1,000. A 2,000 W appliance run for 3 hours uses 2,000 × 3 ÷ 1,000 = 6 kWh.
Where do I find my electricity rate?
Your electricity rate per kWh is printed on your utility bill, usually as a per-unit charge. Rates vary by provider, region, and sometimes by time of day or usage tier.
Does this calculator account for standing charges?
No — it estimates only the variable energy cost (rate × kWh used). Many utility bills also include a fixed daily or monthly standing/service charge that is separate from usage-based cost.
Should electrical work be done by a professional?
Yes. This calculator estimates running cost only; any wiring, circuit or appliance installation work should be carried out by a qualified, licensed electrician in accordance with local electrical codes.
参考文献
- U.S. Energy Information Administration (EIA) — kilowatt-hour definition and residential electricity pricing structure.
- Typical household appliance wattage ranges are drawn from manufacturer spec sheets and standard energy-efficiency guides (e.g., ENERGY STAR product data).