Quick start
- Enter the device wattage from a label, manual, smart plug, or reasonable estimate.
- Enter average hours per day. For a cycling device, use average running time or average watts when you have it.
- Enter the number of days, such as 30 for a rough month or 365 for a year.
- Enter your electricity price per kWh before taxes, fixed fees, or other bill add-ons.
Best uses
Best for one-device estimates, quick appliance comparisons, and deciding whether a device is worth measuring more carefully.
- Estimate appliance energy use.
- Compare a heater, AC, computer, or light over time.
- Turn watts and usage time into kWh.
- Multiply kWh by your local rate.
What this calculator is solving
The Electricity Calculator estimates energy use and cost for one device or appliance. It is useful when you know the wattage, average hours per day, days used, and your electricity price per kWh.
Match each input label on the calculator to the device watts, average hours used per day, number of days in the estimate window, and your electricity rate per kWh.
The formula in plain language
In plain language: The calculator divides watts by 1,000 to get kilowatts, then uses kWh = (watts / 1,000) x hours per day x days. Estimated cost = kWh x rate per kWh. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
The calculator first divides watts by 1,000 to get kilowatts. Then it uses kWh = kilowatts x hours per day x days. Finally, estimated cost = kWh x rate per kWh. A 1,500 W heater running 4 hours a day for 30 days uses 180 kWh; at $0.16/kWh, that is about $28.80.
How to read the answer
Read the kWh line as the energy use, then read the cost line as the estimate from the rate you entered. If the number surprises you, change only one input at a time so you can see whether wattage, hours, days, or rate is doing most of the damage.
- kWh is the energy amount your bill commonly uses.
- Estimated cost multiplies kWh by the rate you entered.
- The rate line reminds you which price was used.
- The same device can look cheap for one hour and expensive over a month, so always check the days field.
Common mistakes to avoid
Most electricity-cost mistakes come from using nameplate watts for a device that cycles on and off, forgetting standby power, mixing watts with kilowatts, or using the wrong price per kWh from the utility bill.
- Do not forget that some devices cycle on and off instead of drawing full wattage all day.
- Do not confuse watts with kilowatts. A kilowatt is 1,000 watts.
- Do not use the full bill total as your price per kWh unless you mean to include fixed charges in a rough way.
- Do not assume standby power is zero. Small idle loads can matter when they run all day.
- Do not use this as a wiring, safety, code, or equipment-sizing sign-off.
Example: space heater for a month
Say a small space heater is rated at 1,500 W and you run it 4 hours per day for 30 days. Your energy price is $0.16/kWh.
The calculator turns 1,500 W into 1.5 kW. Then 1.5 kW x 4 hours x 30 days = 180 kWh. At $0.16 per kWh, the estimate is about $28.80.
That result is useful because it shows why high-wattage heat can become noticeable fast, even when the heater is only used for part of the day.
Example: LED bulb for a year
Now compare a 10 W LED bulb used 5 hours per day for 365 days at the same $0.16/kWh rate.
The calculator uses 0.01 kW x 5 x 365 = 18.25 kWh. The estimated yearly cost is about $2.92.
This is why wattage matters so much. The heater and the bulb may both be common household devices, but their power draw is not even close.
Example: gaming PC or window AC
A 450 W gaming PC used 3 hours per day for 30 days uses 40.5 kWh. At $0.18/kWh, that is about $7.29.
A 900 W window AC used 8 hours per day for 30 days uses 216 kWh. At $0.16/kWh, that is about $34.56.
The AC example is the one to treat carefully because many air conditioners cycle. If the compressor is not running the whole time, average watts may be lower than the label wattage. If the room is very hot or poorly sealed, it may run more often.
Monthly versus yearly estimates
For a quick monthly estimate, 30 days is usually close enough. For a yearly estimate, use 365 days only if the device really runs that pattern all year.
Seasonal devices need a smaller window. A window AC might run hard for 60 to 120 days, while holiday lights, heaters, dehumidifiers, fans, and sump pumps can have very uneven use.
When comparing two devices, keep the days and rate the same so the wattage and hours are the only things changing.
When a device cycles or changes power
Refrigerators, dehumidifiers, heat pumps, air conditioners, pumps, and many heaters do not draw the same watts every minute. They cycle, ramp, or pause.
If you only have the nameplate wattage, treat the answer as a rough upper or planning estimate. For a closer number, use a plug-in power meter, a smart plug with energy tracking, or manufacturer energy-use data.
For computers, TVs, chargers, and network gear, power can also change with brightness, workload, sleep mode, radios, and connected devices. Average watts is better than peak watts when you can get it.
When this guide is not enough
The guide is for appliance-level cost estimates. It does not replace your utility bill, a licensed electrician, product safety instructions, local electrical code, or a home energy audit.
Real bills can include fixed customer charges, taxes, delivery fees, fuel adjustments, tiered rates, time-of-use rates, demand charges, solar credits, and minimum bills. Those details can make the bill total different from a simple kWh x rate estimate.
For electrical safety, wiring, circuit capacity, breakers, extension cords, or equipment installation, use manufacturer instructions and qualified help instead of a cost calculator.
Research and references
These references support the kWh unit, appliance-estimation method, and SI unit wording used in the calculator and guide.
Worked examples for Electricity Calculator
180 kWh, about $28.80
18.25 kWh, about $2.92
40.5 kWh, about $7.29
216 kWh, about $34.56
FAQ in plain language
When should I use the Electricity Calculator?
Use it when your task matches one of these common needs: Estimate appliance energy use. Compare a heater, AC, computer, or light over time. It works best when you already know the measurements, amounts, units, or options the page asks for.
What is the Electricity Calculator doing with my inputs?
In plain language: The calculator divides watts by 1,000 to get kilowatts, then uses kWh = (watts / 1,000) x hours per day x days. Estimated cost = kWh x rate per kWh. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Electricity Calculator inputs mean?
Watts: the device power draw from a label, meter, or reasonable estimate. One kilowatt is 1,000 watts. Hours per day: how long the device actually runs on an average day, not always how long it is plugged in. Days: how many days are in the estimate window, such as 30 for a rough month or 365 for a year. Rate per kWh: your electricity price for one kilowatt-hour before taxes, fixed charges, or other bill fees.
How should I read the Electricity Calculator answer?
Read the headline answer, then check the smaller lines beside it. For everyday tools, those lines usually show the distance, time, cost, units, or setting that made the answer change.
What should I double-check before trusting the answer?
This is an appliance-level estimate from the numbers you enter. Real bills can include taxes, fees, tiered rates, demand charges, standby use, cycling devices, seasonal changes, and variable power draw. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
What is a kilowatt-hour?
A kilowatt-hour is energy use. Running a 1,000 watt device for 1 hour uses 1 kWh. Running a 100 watt device for 10 hours also uses 1 kWh.
How do I estimate monthly electricity cost?
Use 30 or 31 days, enter the device watts, estimate average hours per day, and enter your rate per kWh. For example, 1,500 W for 4 hours a day over 30 days is 180 kWh, which costs $28.80 at $0.16/kWh.
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Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
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