How to use the electricity cost calculator
On the first tab, type the watts printed on the device, the hours a day it runs and the days a week, and the page shows the kilowatt-hours (kWh) it uses a day, a month and a year. To see money, type your own rate from your bill at the top. The page never fills one in, because rates change with your utility, your plan and the hour of the day. The second tab adds up a list of devices and shows which ones account for most of a month, optionally against the kWh and total on a real bill. The third compares two options, such as an old bulb and a new one. The fourth lists typical nameplate watts and converts watts, kilowatts, kilowatt-hours and Btu. Nothing you type leaves your device unless you make a share link.
The formulas
cost = kWh × your rate per kWh
watts = volts × amps (× power factor)
This is the method the U.S. Department of Energy gives for estimating an appliance: watts times hours a day, divided by 1,000, gives kWh a day; times the days used in a year gives kWh a year; times your rate gives the cost. A month is a twelfth of a 365-day year, 730 hours. If a label shows only volts and amps, multiply them to get watts. For motors the real power is lower by a power factor between 0 and 1; if the label gives one, type it. Left blank the page uses 1, which can overstate a motor. Volts and amps here means ordinary single-phase household loads.
A worked example
A portable heater is labeled 1,500 watts and runs 4 hours a day, every day. That is 1,500 × 4 ÷ 1,000 = 6 kWh a day, 2,190 kWh a year, or 182.5 kWh a month. The page shows exactly that when it opens, and no money, because it has no rate. To show the money step, suppose a rate of 10 cents per kWh. That is a made-up round number to keep the arithmetic easy, not an average and not a guess about your bill. Then the heater costs $0.60 a day, $18.25 a month and $219 a year. Type your own rate and the same steps use yours.
Kilowatts and kilowatt-hours are different
A kilowatt (kW) is a rate: how fast a device uses energy right now. A kilowatt-hour (kWh) is an amount: that rate sustained for an hour, like the speed of a car and the miles driven. A 1,500 watt heater is 1.5 kW whether it runs a minute or all night, and uses 1.5 kWh in an hour. Your bill charges for kWh, so a high-wattage device that runs for minutes can use less than a small one that never turns off.
Cycling devices and standby power
The label shows the most a device draws, and many do not draw it all the time. A refrigerator's compressor cycles, and a heater with a thermostat stops once the room is warm. The box "percent of the time at full power" is for that: 50 means it draws its label watts half the time it is on. The best way to know is a plug-in power meter over a day or a week. Devices also draw a little when they look off. Type those standby watts and the page counts them for every hour the device is not in use.
Your rate, fixed charges and demand charges
Look at the electricity part of your bill for a price per kWh. It is often split into the supply (the energy) and the delivery (the wires and the utility's costs); add the two to get one number. A flat monthly customer charge does not change with use, so it is not part of a rate per kWh. Some plans charge different rates at different hours, which the time-of-use box handles for one device. A few plans, mostly for businesses, also charge for your highest kW at one time, a demand charge; this page does not model it. Your utility's rate sheet is the authority.
Where the numbers come from
The formula is the U.S. Department of Energy's, from its Energy Saver article on estimating appliance and home electronic energy use. The typical watts come from Virginia Cooperative Extension's publication of the same name (BSE-301NP, February 2020): nameplate figures and ranges, not measurements of your devices. The conversion of 1 kWh to about 3,412 Btu is from the U.S. Energy Information Administration, and 1 kWh = 3.6 megajoules is a definition; the converter carries more digits of the Btu figure from the standard Btu definition. No building code applies. This is an estimate, not an energy audit, and it does not size wiring or breakers.
What people use it for
- Finding what one space heater, window unit or dryer adds to a bill.
- Checking a bill by adding up what runs in the house.
- Comparing a new bulb or appliance with the old one, with payback.
Common mistakes
- Forgetting to divide by 1,000. 1,500 watts for 4 hours is 6,000 watt-hours, which is 6 kWh.
- Using cents as dollars. 12 cents per kWh is 0.12 dollars. The unit box next to the rate handles it, and the page warns if a number looks like the other unit.
- Treating the label as average use. A cycling device draws less over a day than its label.
- Leaving out standby. Small amounts add up over 8,760 hours a year.
Questions
How do I calculate the cost of running something?
Multiply its watts by the hours it runs and divide by 1,000 to get kWh. Multiply the kWh by the price per kWh on your bill. Type your rate at the top of the page and it does both steps.
What is the difference between a watt and a kilowatt-hour?
A watt measures power, how fast energy is used. A kilowatt-hour measures energy, how much was used: 1,000 watts for one hour.
How do I find the watts if only amps are printed?
Multiply volts by amps. Most household outlets in the US are 120 volts; large appliances like dryers use 240. Choose "volts and amps" in the first tab.
Does it include taxes and fees?
Only if your rate does. On the bill tab, type your bill's kWh and total for your all-in cost per kWh.
Can I share my result?
Yes. Share makes a link with a picture for Facebook, a link you can copy, and a short video for TikTok made on your device. The link keeps only the watts, count, hours, days, percent and standby watts, and your rate only if you typed one. It never keeps device names, your bill or prices.