kWh Cost Calculator — Running Cost, Any Load
Watts tell you how hard something pulls; kilowatt-hours tell you what it costs. The conversion is simple — watts ÷ 1,000 × hours × your rate — but the number it produces is often the most persuasive figure in a quote. Enter the load, its real run time, and the rate off an actual bill.
Calculate running cost
Take the rate from an actual bill — divide the total charge by the kWh delivered, which captures taxes and delivery fees the headline rate leaves out.
Monthly cost
1,500 W running 4 h/day for 30 days uses 180 kWh a month.
Per day
$1.02
Per year
$367
See the breakdown
Energy component only. Real bills add fixed connection charges, taxes, and delivery fees, and thermostatted appliances cycle rather than running continuously.
The formula, explained in plain English
Power vs energy
Watts is a rate; kilowatt-hours is a quantity. A 1,000 W load for one hour and a 100 W load for ten hours cost exactly the same — the bill only counts energy.
Run hours dominate
A 150 W refrigerator running 24/7 costs more than a 1,500 W heater used an hour a day. When an estimate looks wrong, the run-time assumption is almost always why.
Commercial bills add demand
Commercial tariffs charge for peak kW or kVA on top of energy. Reducing peak draw can save more than reducing total consumption.
Use the effective rate
Divide the whole bill by the kWh delivered. That captures taxes, delivery, and fixed charges, and is usually well above the advertised energy rate.
Worked examples
A heater, an always-on load, and an EV — three very different shapes.
1,500 W space heater, 4 h/day
30 days a month at $0.17/kWh. The defaults above.
× 30 days = 180 kWh → $30.60/month
× 12 = 2,160 kWh → $367/year
Result: $367 a year to heat one room with resistance heat. The same heat from a heat pump at COP 3 would cost roughly a third of that, which is the electrification argument in one number.
150 W refrigerator, 24 h/day
A tenth the power, but always on.
× 30 = 108 kWh → $18.36/month → $220/year
Result: a 150 W load costs 60% as much as a 1,500 W load, because it runs six times as long. Run hours matter more than wattage.
48 A EV charger, 4 h/day
11,520 W (48 A × 240 V), charging four hours a night.
× 30 = 1,382 kWh → $235/month → $2,820/year
Result: daily full-rate charging is a serious load — comparable to a whole small household. This is also why time-of-use tariffs and overnight rates change the answer dramatically; rerun it with your off-peak rate.
Common appliance running costs
At $0.17 per kWh and 30 days a month, using the run hours shown. Change either input above to match your situation.
| Appliance | Watts | h/day | kWh/day | Per month | Per year |
|---|---|---|---|---|---|
| LED bulb (10 W) | 10 | 5 | 0.05 | $0.26 | $3 |
| Desktop computer | 200 | 8 | 1.60 | $8.16 | $98 |
| Refrigerator (averaged) | 150 | 24 | 3.60 | $18.36 | $220 |
| Window air conditioner | 900 | 8 | 7.20 | $36.72 | $441 |
| Space heater | 1,500 | 4 | 6.00 | $30.60 | $367 |
| Electric water heater | 4,500 | 3 | 13.50 | $68.85 | $826 |
| Clothes dryer | 5,000 | 1 | 5.00 | $25.50 | $306 |
| EV charger (48 A) | 11,520 | 4 | 46.08 | $235.01 | $2,820 |
Sources & standards: electricity price per kilowatt-hour from the U.S. Energy Information Administration Electric Power Monthly. The $0.17/kWh default is a US residential average; state rates range from roughly $0.11 to over $0.30. Figures cover the energy component only and exclude fixed charges, taxes, delivery fees, and demand charges.
Frequently asked questions
Common questions about kilowatt-hours and electricity cost.
How do I calculate electricity cost?
Convert watts to kilowatts, multiply by hours to get kilowatt-hours, then multiply by your rate: cost = (watts ÷ 1,000) × hours × $/kWh. A 1,500 W heater for 4 hours uses 6 kWh; at $0.17 per kWh that is $1.02 a day.
What is a kilowatt-hour?
One kilowatt of power drawn for one hour. It is a unit of energy, not power — the distinction that matters on a bill. A 1,000 W appliance running one hour and a 100 W appliance running ten hours both consume exactly 1 kWh and cost the same.
What's the average cost per kWh?
US residential electricity averages around $0.17 per kWh, but the spread across states is enormous — roughly $0.11 in the cheapest states to over $0.30 in Hawaii and parts of the Northeast. Commercial rates are usually lower per kWh but add a demand charge based on peak kW or kVA. Take the rate off an actual bill rather than using a national average: divide the total by the kWh delivered to capture taxes and fees too.
Where do I find an appliance's wattage?
On the nameplate or rating label — usually on the back, base, or inside the door. If it gives only amps, multiply by the voltage: 12.5 A at 120 V is 1,500 W. If it gives volt-amperes, that is close enough to watts for cost estimating on resistive loads, but will overstate cost on motors and electronics with a power factor below 1.0.
Why is my actual bill higher than this estimate?
Usually three reasons. Appliances with thermostats or compressors cycle rather than running continuously, so real run hours differ from the nameplate assumption. Bills include fixed connection charges, taxes, and delivery fees on top of the energy rate. And tiered or time-of-use tariffs mean marginal kWh can cost far more than average. This calculator gives the energy component only.
Does this work for an EV charger or a heat pump?
It works, but be careful with the hours. An EV drawing 11.5 kW for 4 hours uses 46 kWh — about $7.82 at $0.17. For heat pumps, the nameplate is the maximum draw, not the average: a modern inverter unit modulates and may average a third of it over a day. Use metered data if you have it. For HVAC operating cost specifically, the HVAC Operating Cost Calculator accounts for efficiency ratings.
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