Quads are higher at 185W and 550W respectively.
http://support.apple.com/kb/TA24037
Sorry, i only based my calculations on the machines with the smallest power requirements (just to give the guy a break).
But you are a 100% right at the fact that there would be some serious cooling involved. Running a cooling-room for 24 hours a day isn't exactly viable.
I don't know how serious OP is, but I think the idea was to offset the electricity used for this hobby, not power them from panels in real time. I certainly agree that you'll not do that on this budget. He mentioned that he will use the cluster sparingly.
Then you would require a storage device for said electricity, preferably some kind of battery. The big problem in that case would be to find one that could store the energy without having to send off excess power (and it will certainly cost a lot).
If the PM's are going to be plugged in during the usage-intervalls, the battery needs to be able to hold a charge that will keep them "running" during the hours when they are turned off.
And the meaning of a cluster is to process processor-intensitive tasks, this means that they would without a question all reach the maximum power consumption (450 watts on the G5 Quad) and will require a lot more energy than i noted in my previous comment (and thus the overall idea fails).
Or he might just use it to watch movies, but that would be a big waste of electricity and time.
Dual G5 2.5Ghz XServes are in the same price range as the Quad G5 Power Mac. The XServe is also much louder and will require it's own room if you mind the noise. Drive sled's are getting harder to find and it lacks PCIe slots.
Don't get me wrong, i have been looking into doing this for PPC virtual machines for a long time. I originally wanted XServes, but for the same price i can get 2 additional cores, PCIe, and standard storage attachment in the PowerMac.
G5 Xserve Cluster? Man, that would make the neighbors complain!
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OP, if you wish to create a cluster with a budget, at least use Intel machines (Mini (early 2006 idle: 23 watt)) or PPC systems that consumes
less watts than a G5 (Mini (2005 idle: 32 watt)).