An appliance label usually tells you power, not price. To estimate what it costs to run, convert watts into kilowatt-hours, multiply by the time used, and apply your electricity rate. This guide explains the calculation, shows how to handle daily and monthly use, and points out why real bills can differ from a simple nameplate estimate.
The electricity-cost formula
The core formula is cost = power in watts ÷ 1,000 × hours used × price per kilowatt-hour. Dividing by 1,000 converts watts into kilowatts, and multiplying by hours produces kilowatt-hours. If a 1,500-watt space heater runs for 3 hours at $0.18 per kWh, the estimate is 1,500 ÷ 1,000 × 3 × $0.18, or $0.81 for that session.
Use the electricity-cost calculator when you know the appliance wattage, expected runtime, and rate. If the label gives amps and volts instead, watts are approximately volts × amps for a simple device, although motors and power factor can make the real draw different. For a first estimate, use the rated wattage; for a decision that matters, measure actual consumption with a suitable meter.
From one use to a monthly estimate
To estimate a month, calculate one day and multiply by the number of days used. A 100-watt fan used for 8 hours each day consumes 0.1 × 8 = 0.8 kWh per day. At $0.20 per kWh, that is $0.16 per day or about $4.80 over 30 days. The same method works for a refrigerator, computer, aquarium pump, or any other device when the runtime is reasonably predictable.
Runtime is often the biggest assumption. A heater may cycle on and off instead of drawing full power continuously. An air conditioner’s compressor may run only part of the hour, while a laptop’s power draw changes with workload and charging state. If you are unsure, calculate a low and high scenario rather than presenting one precise-looking number.
Read your electricity rate carefully
The rate on a bill may include more than the energy charge. Some utilities have delivery charges, fixed fees, time-of-use rates, demand charges, taxes, or tiered pricing. For a rough appliance comparison, use the energy rate. For a household-budget estimate, use an effective rate that reflects the portion of the bill you are trying to explain, and state which fees are included.
Peak and off-peak prices can change the best time to run a device. The calculator can compare the same wattage and runtime at two rates, making the tariff difference visible. Do not use a monthly average rate to predict a specific high-load period if your utility charges differently by time or season.
Appliances with heating or cooling loads
Heating and cooling devices deserve extra care because they can use a lot of energy and may cycle. A space heater rated at 1,500 watts can cost more than a small fan simply because its power is fifteen times higher. An air conditioner may list cooling capacity separately from electrical input, so use the electrical input when estimating cost rather than the cooling output alone.
For these appliances, compare settings and duty cycles. Lowering a thermostat, improving insulation, cleaning filters, or reducing runtime may matter more than chasing a small difference in the advertised wattage. Use the dedicated appliance, air-conditioner, or space-heater calculators when you want a more focused set of assumptions.
Worked example: compare two devices
Imagine a 60-watt fan used for 10 hours per day and a 1,500-watt heater used for 2 hours per day. At $0.20 per kWh, the fan costs 0.06 × 10 × $0.20 = $0.12 per day. The heater costs 1.5 × 2 × $0.20 = $0.60 per day. Over 30 days, those estimates are $3.60 and $18.00. The heater runs for fewer hours but still costs much more because its power draw is far higher.
Now test a realistic duty cycle. If the heater is on a thermostat and draws full power only half the time, its estimated cost may be closer to $9 per month. That does not make the nameplate rating wrong; it means the real runtime is different. Write down the assumption beside every estimate so a later comparison remains understandable.
Use the estimate to make a decision
Calculate the cost of the current option and the alternative using the same rate and period. Then consider purchase price, maintenance, comfort, safety, and expected life. Electricity cost is one input, not the whole decision. If the result is surprisingly high, check the wattage, decimal placement, runtime, and whether you entered cents per kWh where the calculator expects dollars. Saving the inputs alongside the result makes it easier to update the estimate when your rate or routine changes. It also gives you a useful record when comparing a new appliance with the one it replaces, especially if the two devices have different duty cycles.