How to use
- Read watts on the device label or the power supply. If you only know kilowatts, multiply by 1,000 before you type.
- Enter hours for the period you care about. Five hours a day for 30 days is 150 hours.
- Enter the energy price from your bill, in dollars per kilowatt-hour.
- Press Calculate. Cost is the hero. Energy in kWh is beside it.
How it's calculated
Kilowatt-hours = watts ÷ 1,000 × hours. Cost = kilowatt-hours × price per kWh.
A device that cycles, such as a refrigerator, does not draw its nameplate watts the whole time. Use an average wattage if you have one. The page will not invent a duty cycle.
Worked example
A 60 watt bulb run for 150 hours uses 60 ÷ 1,000 × 150 = 9 kWh. At $0.16 per kWh the energy costs $1.44. A 1,500 watt heater for 2 hours uses 3 kWh, which is $0.48 at that same price.
Assumptions
Constant watts for the hours you enter. No demand charge, no taxes, and no time-of-use split unless you already picked a single price that represents them.
FAQ
Where do I find the price per kWh?
On the energy line of the bill. If several charges apply per kWh, add the ones that scale with energy and enter that sum.
What if the label says amps?
Watts = volts × amps for a simple resistive load on a known voltage. This page does not ask for amps, so do that multiplication first.
Does standby power count?
Only if you include those hours and that smaller wattage. Run a second calculation and add the costs.
Is a kilowatt-hour 1,000 watts?
A kilowatt-hour is 1,000 watts used for one hour, or any combination with the same product, such as 500 watts for two hours.