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How to Measure Actual Computer Power Usage, Wattage, and Cost
Sep 8, 2016Computer HardwareComments (2)
If you're curious how much power your computer uses at the outlet, you'll have to get a measuring tool. You can get a rough estimate by adding up published specs for each component, but that won't be too accurate and you won't be able to determine load-vs-idle power usage.

Kill-A-Watt Power Meter
The tool you'll need to measure your power usage is the P3 Kill-A-Watt power meter. It looks like it's from the 90's, but it's still the best one for light residential usage. You simply plug it into the wall, then plug whatever you want to measure into it. Anything that runs on 115V AC power with a standard US wall plug can be measured.

My PC's Power Usage
Here are the parts and specs for my PC:
  • Seasonic 430W Power Supply
  • MSI Pro-VD Motherboard
  • Intel Core i3-6100 CPU
  • GeForce GTX 950 GPU
  • 16GB DDR4 Memory
  • 256GB Solid State Drive
  • 1TB Hard Disk Drive
(View the PC build guide here.)

Here is its power usage at various states:
  • Off, plugged in: 0.5-2 watts
  • Booting up: 60-70 watts
  • Idle desktop: 40-45 watts
  • Opening programs: 50-60 watts
  • KSP main menu: 70-75 watts
  • KSP space center: 120 watts
  • KSP ship in interplanetary space: 70-80 watts
  • Prime95 blend: 92 watts
  • Prime95 large FFT: 97 watts
  • Windows 7 Experience Index test: 90-170 watts

I was able to get my PC to consume up to 170 watts peak during the Windows Experience Index test. I could probably get it higher if using a stress tester that maxed out GPU and CPU at the same time. Considering PCPartPicker estimates my theoretical peak power usage at 228 watts based on parts specs, that sounds about right.

Calculating How Much Power Costs
Power usage is measured in kilowatt hours (kWh). Your power company will usually charge a certain amount per kWh used, something like $0.10 or $0.20 per kWh (it varies widely from area to area). Look on your last electricity bill to see what they're charging per kWh. If you want to see state averages, check out this chart.

Example: If you pay $0.12 per kWh, that means it costs you $0.12 to run something for one hour that consumes 1000 watts. A typical microwave uses 1500 watts while running, so if you ran it for one hour straight (that's a lot of hot pockets!) it would cost you $0.18, assuming you pay your energy company $0.12 per kWh.

Once you have your kWh cost and your PC's power usage (in watts), you can now calculate how much it costs to run. Divide its watt usage by 1000, then multiply that number by your kWh cost. That will give you the cost of running it for one hour at that specific wattage.

Example: Say your PC uses 350 watts, including monitor, while playing a graphically-intensive game, and your kWh cost is $0.14. If you were to play for one hour straight, it would cost you:

350 / 1000 * 0.14 = $0.049
That's roughly 5 cents. It doesn't sound like much, but can add up. Say you play on average 2 hours a day, every day of the year. That's $35.77 per year.

What if you have a power-efficient PC that uses only 45 watts at idle, but leave it on for 8 hours a day? That's 5 cents per day, or $18.40 per year, just to sit idle.
Comments (2)
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Carl Metzler   Oct 03, 2019
I doubt the accuracy of a Kill-A-Watt meter when measuring non-sinusoidal current waveforms that typically happen on electronic equipment like computers. I wrote the mfr. and asked about that, but all they wrote back was "The instrument is accurate to 0.2%," which is the same thing the spec says. I have one and hooked it up to a Vornado tower fan that has electronic controls such that it still draws some power when it's supposedly off and plugged in. In "off" mode, the meter showed the fan drawing 1 watt and 10 amps (!) with a power factor of 0.1. Those numbers are certainly self-consistent. But if the fan was really drawing 10 amps, why didn't the 2.5 amp fuse in its plug blow?
Margarida Costa   Apr 06, 2017