It assumes 250 work days a year, with 7 hours of productivity applications and one hour of maximum performance applications. (That's 2,000 hours of use yearly-Dell apparently assume a 2-week vacation, but you can change those parameters, as well as all others.) The price of electricity is assumed to be 10 cents per kilowatt hour, which is indeed about the national average.
It only works with Dell office configurations (OptiPlex and Vostro desktops, and Latitude laptops.) There are countless permutations, but I played with it to get some rules of thumb, which would hopefully hold true outside Dell.
Basically, I found that a respectable desktop system will use about $25 worth of electricity yearly. However, there is considerable variation, and anyone thinking of order hundreds of identical systems might want to play with the calculator and explore the impact of various configuration decisions.
For instance, having 8 gigabytes of RAM consumed 4.8 kilowatt-hours yearly than having 2 gigabytes. Having 16 gigabytes of RAM consumed 14.5 kilowatt-hours yearly than having 2 gigabytes.
Having one optical drive adds 7.7 kilowatt hours, while having two adds 16.4 kilowatt hours.
Adding a solid-state hard drives adds three kilowatt hours, and looks like a good deal from that angle. (The purchase price, of course, makes it look more complicated.) Adding a 5400 rpm hard drive adds 9.2 kilowatt hours while a 7200 rpm drive adds 10 kilowatt hours, and a 10000 rpm drive adds 11.6 kilowatt hours.
Having a Core 2 Quad CPU adds 3.8 kilowatt hours compared to having a Core 2 Duo.
Doubtless you could go on for hours-and if there is big money involved maybe you should. Keep in mind that the calculator is based on Dell's stated assumptions. Field testing with a power meter is the final answer.
Finally, you'd think that Dell would be shy about offering such a calculator, lest is scare customers away from heavy configurations. But apparently that horse is out of the barn-people automatically want all the power they can get.
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