In this day and age with most longtime Mac users moving to Intel systems but still having perfectly good PowerPC hardware in the closet, many ask, "What can I do with those old computers? I'm not really a modder and don't need a fish tank. Is there anything close to their original purpose I can use them for?" The answer most people come up with, if it isn't eBay, is some sort of server: a file server, music/media, or even a web server. And given Power Macs have multiple drive bays, they'd seem ideal for this task. There's just the one issue of power consumption. Since these computers will generally always be on, it's a good idea to look at their wattage efficiency to see which models would be best served being servers of the served.
I think everybody knows that G5 Power Macs suck a lot of power. They give off enough heat, so much that they're sometimes referred to as space heaters. In fact, most G5s idle (on but no processor activity) at around 150 watts. As a comparison, most laptops and minis idle at around 20-30 watts and even iMac G5s can idle at less than half the Power Mac's wattage with the screen asleep. So G5s aren't ideal as servers, especially since they're limited to two drive bays.
So let's go back a generation. I assumed the MDD Power Macs would be much more efficient, at least in a processor idle state, since they're equipped with G4s rather that G5s. But this article at Apple's website was a real eye-popper, and not in a good way. The MDDs are practically just as power hungry as G5s. The single processor 1.25 GHz G4 which was the last Mac to boot OS 9 wasn't so bad. It idled at 80 watts, but the others, particularly the dualies, all idled at 130-150 watts. That's asking for a pretty big hit in the wallet when the power bill comes.
You have to go back to the Quicksilvers and Sawtooths to find Power Macs that idle in the 50-75 watt range, and it also so happens that those models are much more reliable than what followed. So if you're looking to utilize a server with multiple drive bays, those might be your best options.
One last note, as a way to conserve power you can run G5 processors as "reduced" in the Energy Saver preferences pane. That might save you a little electricity. Also, G4s can utilize Nap Mode in CHUD Tools. I'm not intimately familiar with this, but googling it will give you plenty of links.
*Dan Knight in comments points out the "big drive" disadvantage pre-Quicksilver Power Macs have with not supporting drives greater than 128 GB without a third party card. Also, in this xlr8yourmac page users report that Nap Mode reduces watt usage in MDDs up to 35% to 40% at idle. Interesting.