Turn a wattage label, the hours of use and your electricity rate into a monthly and yearly cost, then decide which changes are worth making. Flags the 4 mistakes that most often produce a plausible-but-wrong answer.
Key takeaways
- Cost equals kilowatts times hours times price, and each of the three can be measured.
- A plug-in meter beats the label for devices that cycle on and off.
- Standby loads run for 8,760 hours a year, so small wattages add up.
- A blended rate from your whole bill is closer to reality than the energy-only rate.
- Rank devices by monthly cost first, since a few large loads usually drive the bill.
Cost is power times time times price
Every running cost has the same three parts. Power is the wattage of the device, time is the hours it runs, and price is what you pay for each kilowatt-hour. Divide the watts by 1,000 to get kilowatts, multiply by the hours to get kilowatt-hours, and multiply by the price. A 1,500 watt heater run eight hours a day for 30 days uses 360 kilowatt-hours.
Find the wattage on the label or in the manual
Most appliances list their power on a rating plate, in watts or in volts and amps, which multiply to watts. Devices that cycle, such as a refrigerator or a thermostat-controlled heater, draw their full wattage only part of the time. For those, a plug-in energy meter measures real use over a day or a week and beats any estimate from the label.
Hours of use are where estimates go wrong
People overestimate the hours of a device they rarely use and underestimate those that never switch off. Count standby devices, chargers and always-on boxes too. A device that draws a few watts around the clock runs for 8,760 hours a year, so a small wattage adds up. Use the real hours from a week of observation, not a guess.
Find your real electricity rate
The price per kilowatt-hour on a bill often differs from the all-in cost. Utilities add delivery charges, fixed fees and taxes, and some charge different rates by tier or by time of day. For a close estimate, divide the whole bill by the kilowatt-hours used to get a blended rate, or use the marginal rate that applies to the next kilowatt-hour.
Rank devices before you replace them
Once each device has a monthly cost, sort the list. A few devices, usually heating, cooling, hot water and the biggest motors, account for most of the bill. Replacing a small load rarely changes the total, while a change to the biggest one does. Compare the yearly saving with the price of the replacement to find the payback.
Lighting is the classic easy saving
Swapping older bulbs for LEDs cuts the watts for the same light, and the saving grows with the hours the bulbs burn. Twenty 60 watt bulbs replaced by 9 watt LEDs, on three hours a day, saves about 1,117 kilowatt-hours a year. The bulbs in rooms with the longest hours pay back first, so start there.
Electric vehicles use the same arithmetic
An electric car's cost per mile is its energy use per 100 miles times the electricity price, adjusted for the energy lost while charging. At 30 kilowatt-hours per 100 miles, 10 percent charging loss and 17 cents a unit, the cost is under six cents a mile. Home rates, time-of-use plans and public fast chargers can change that number a great deal.
Solar changes the price side of the formula
A solar array produces kilowatt-hours, which offset the ones you would buy from the utility. Output depends on the rated power, the peak sun hours where you live and the losses in wiring, heat and dirt. Payback is the cost after incentives divided by the yearly savings, and it moves with your electricity rate and with how much of the output you use yourself.
Heating and cooling need a different lens
Heating and cooling costs depend on the weather, the building and the equipment's efficiency, not only on a wattage label. A heat pump moves more heat than the electricity it uses, so its cost per unit of heat is lower than resistance heating. For these systems, compare efficiency ratings and use a load calculation, and treat a simple watts times hours estimate as a rough check.
Check the estimate against your bill
After you total the estimated costs, compare the sum with the usage on a recent bill. If the two differ by a lot, something is missing or overstated. A month-by-month view also shows seasonal loads. Estimates guide decisions best when they agree with the bill you actually pay.
Worked with real numbers
What this looks like in the electricity cost calculator
The guidance above is easier to judge against figures. Using the electricity cost calculator worked example, moving electricity price per kwh from $0.14 to $0.20 changes the cost for the period from $50 to $72.
| Electricity price per kWh | Cost for the period | Energy used (kWh) | Cost over a full year at this rate |
|---|---|---|---|
| $0.14 | $50 | 360 | $613 |
| $0.15 | $54 | 360 | $657 |
| $0.17 | $61 | 360 | $745 |
| $0.19 | $68 | 360 | $832 |
| $0.20 | $72 | 360 | $876 |
Open the Electricity Cost Calculator to use your own numbers →
The inputs behind those figures
| Input | Value | Definition |
|---|---|---|
| Power draw (watts) | 1,500 | Printed on the appliance's label or in its manual. |
| Hours used per day | 8 | Enter the hours used per day used in this calculation. |
| Electricity price per kWh | $0.17 | Found on your bill, in dollars per kilowatt-hour. |
| Days of use | 30 | Enter the days of use used in this calculation. |
What goes wrong
Common mistakes
Each of these produces an answer that looks reasonable, which is why they survive review. To catch them in how to work out what an appliance costs to run, rerun the electricity cost calculator with a different assumption and check whether the result moves in the direction the guidance predicts, since an error that survives that test is usually in one of the inputs and not in the arithmetic.
| The mistake | Why it misleads | Do this instead |
|---|---|---|
| Using the rated wattage as the average draw | Thermostat-controlled devices run their full wattage only part of the time, so the label overstates their cost. | Measure real use with a plug-in meter over several days. |
| Counting only the energy charge in the rate | Delivery fees, taxes and fixed charges are part of what you pay, and leaving them out understates every cost. | Divide the whole bill by the kilowatt-hours used to get a blended rate. |
| Forgetting standby devices | Chargers, boxes and always-on devices run all year and rarely appear on anyone's list. | Add them with their hours set to 24 a day and see the yearly total. |
| Replacing small loads first | A saving on a device that costs a few dollars a year barely changes the bill. | Start with the largest monthly costs, usually heating, cooling, hot water and big motors. |
People also ask
Frequently asked questions
How do I calculate the cost of running an appliance?
Divide its watts by 1,000, multiply by the hours it runs, and multiply by your price per kilowatt-hour. The electricity cost calculator does the sum and shows the yearly figure.
Why is my estimate different from my actual bill?
Devices that cycle draw less than their label, standby loads are often missed, and the rate on the bill includes fees. A plug-in meter and a blended rate close most of the gap.
What is a kilowatt-hour?
The energy used by a 1,000 watt device running for one hour. It is the unit a utility bills in.
How much does it cost to charge an electric car?
Multiply the energy the car uses per mile by the miles driven and your electricity price, and add the energy lost while charging. The EV charging cost calculator shows the monthly and per-mile figure.
Are LED bulbs really worth it?
Yes for bulbs that burn many hours a day, since the saving scales with hours. A bulb that is on for a few minutes a day may take years to repay its price.
How long does a solar system take to pay back?
Divide its cost after incentives by the yearly savings. Your electricity rate, the sun where you live and how much you use yourself all change the answer.
Do time-of-use rates change the calculation?
Yes. Use the rate that applies at the hours a device runs, since the same appliance can cost much more in a peak period than overnight.
How much does a device on standby cost?
Multiply its standby watts by 24 hours, 365 days and your price per kilowatt-hour, after dividing by 1,000. A few watts around the clock adds up to a few dollars a year for each device.
What is a blended electricity rate?
Your whole bill divided by the kilowatt-hours used. It includes delivery charges and fees, so it is closer to the real cost of an extra kilowatt-hour than the energy-only rate.
Which input moves the cost for the period most in the electricity cost calculator?
Ranked by how far each moves the cost for the period across the range tested: hours used per day ($15, 25%), electricity price per kwh ($14, 24%), power draw (watts) ($12, 20%) and days of use ($12, 20%).
If I double hours used per day in the electricity cost calculator, does the cost for the period double?
Doubling it from 8 to 16 takes the cost for the period from $61 to $122, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 4 gives $31.