Measuring computer energy consumption

August 10, 2007

I wrote a Yahoo! widget that displays the energy cost of running a computer. Based on Google’s paper on data center energy consumption, it uses CPU load as a good approximation of energy usage. You can calculate your computer’s peak power requirement by adding up all the components in your system.  Since your power supply loses some energy to heat, you should divide by it’s efficiency rating (mine’s 85%, but most computers are ~60%). This total number will be significantly higher than the true peak power load because manufacturers list very conservative (i.e. high) numbers. Google found their server ran at 60% of the “nameplate” value. I’d say take 80% of your total to get closer to a real peak power number.

At 0% CPU, a computer still consumes 50% of peak power. It’s linear from 0% to 100% CPU, so it’s easy to compute how much power is consumed: (50+(CPU load/2))% of peak power(W/hr). Multiply this by your cost per kW/hr to get the energy cost. You can also compute how much pollution was generated to provide you with that energy using this table: http://www.eia.doe.gov/oiaf/1605/e-factor.html. The point of all this is to illustrate that the energy required to run your computer is a significant cost per year. If my computer sits idle all year, it costs about $70 $200 to power it. [edit: I misread my power bill]

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