Watts are a rate, kWh is an amount
A watt measures how fast something uses energy right now. A kilowatt hour (kWh) measures how much it used over time: 1,000 watts for one hour, which is also 1,000 watt hours or 3.6 megajoules. A 100 W device that runs 10 hours uses 1 kWh, and a 60 Wh laptop battery holds 0.06 kWh.
The two get mixed up often. A kW is a rate, like speed. A kWh is an amount, like distance. "kW per hour" isn't a unit.
The cost formula
Cost takes two steps:
- Watts times hours, divided by 1,000, gives kWh.
- kWh times your price per kWh gives the cost.
A 100 W home server that runs all day uses 876 kWh a year. At $0.18 per kWh, the site's reference rate, that's $157.68 a year or $13.14 a month. The same 100 W for 4 hours a day uses 146 kWh and costs $26.28 a year.
Where the numbers go wrong
- Rating isn't use. A power supply labeled 850 W or a chip with a TDP sets a limit, not the draw. Many devices spend most of their time below their rating, so use average watts or the utilization setting in the Power Cost Estimator.
- Standby counts. A device that's left on or in rest mode runs all year, and the small numbers add up.
- Your rate isn't the site's. Bills quote a price per kWh that varies by country, utility and sometimes time of day. Swap in the rate from your own bill.