How this instrument works
Every appliance's running cost comes down to three numbers: how much power it draws in watts, how many hours it runs, and what you pay per kilowatt-hour (kWh) — the standard billing unit utilities use, equal to 1,000 watts sustained for one hour. Multiply power by time to get energy in kWh, then multiply by the rate, and the result is the cost for that stretch of use.
This is the general-purpose version of that calculation: enter any wattage and any number of hours, from a quick single session to a full month's worth of running time, and get the cost for exactly that stretch. It's the same arithmetic a utility meter effectively performs continuously, just applied to one appliance and one time window at a time instead of an entire household's whole billing period.
The one input worth double-checking is wattage — nameplates sometimes list amps instead of watts, especially for motors and older appliances, in which case multiplying amps by voltage first (a separate calculation) gets you the watts figure this instrument needs.
- Enter Appliance power (W) — the wattage from the appliance's nameplate, manual, or a plug-in power meter reading.
- Enter Hours used — the number of hours the appliance runs during the period you're costing out.
- Enter Electricity rate ($/kWh) — your utility's rate per kilowatt-hour, found on a recent bill (a flat average rate works fine even if your plan has tiered pricing).
- Read Cost ($) — the electricity cost for that appliance over that many hours.
- For a recurring monthly estimate, multiply daily hours by roughly 30, or run the hours-per-day figure through and scale the result yourself.
Worked example — a 1,500 W heater for 3 hours
Enter 1500 into Appliance power (W), 3 into Hours used, and 0.15 into Electricity rate ($/kWh) — a 1,500 W space heater run for 3 hours at a typical US residential rate of 15 cents per kWh. Cost reads (1500 ÷ 1000) × 3 × 0.15 = 1.5 × 3 × 0.15 = $0.675.
That 67.5-cent figure is for this one 3-hour stretch specifically; running the same heater every evening for a month at similar duration adds up to roughly 30 × $0.675 ≈ $20.25 — which is exactly why space heaters, run for hours at a time through a whole winter, show up as a noticeable line on a heating-season electricity bill.
Questions
My appliance's label only lists amps, not watts — what do I enter?
Multiply amps by your circuit's voltage (120 V is standard in the US, 230 V in the UK and much of the EU) to get watts — amps × volts = watts by Ohm's law. This site's appliance wattage calculator does that conversion directly if you'd rather not do it by hand before coming back here.
What is a kilowatt-hour, and why does the bill use it instead of watts?
A kilowatt-hour is the energy used by a 1,000-watt load running for one hour — utilities bill in kWh because it's a unit of total energy consumed, not just instantaneous power, and that's what actually costs money over time. A 100 W bulb left on for 10 hours uses exactly the same 1 kWh as a 1,000 W device run for 6 minutes.
Where do I find my actual electricity rate?
Look at a recent utility bill for the per-kWh rate, sometimes listed as a single flat rate and sometimes broken into tiers, time-of-use periods, or a supply charge plus a delivery charge that should be added together. A single blended average rate is close enough for most appliance-cost estimates; exact billing structures vary a lot by utility and plan.
How is this different from the single-use electricity cost calculator on this site?
Same formula — this page is built for general running-cost questions across any duration, from a few hours to a full month's worth of use, while the single-use version is framed specifically around costing out one individual session (leaving a lamp on overnight, one load in the dryer) as a standalone question, rather than an ongoing or recurring estimate.
Does this account for appliances that cycle on and off, like a fridge or AC?
Not directly — Hours used should reflect actual running time, not the time the appliance is simply plugged in or turned on at the wall. A refrigerator's compressor might run roughly a third to half of any given hour depending on the model and ambient temperature, so its effective 'hours used' is less than 24 even though it's never switched off.