Just putting this out there, would you not get better efficiency if you run the S5 off 2 PSU's, e.g a 550W for each blade than run the whole rig off a single 750W/850W/1100W PSU?
Depends. Generally within the same product line a 550W running at 50% load will be about as efficient as a 1100W running at the same 50% load. More practically, the 550W units would tend to be easier to find and cheaper, but often the cheap ones you find aren't as efficient or well built as the 1100W units. You then also have another full ATX supply to deal with, and if you have more than a couple of these it gets to be a pain. I actually thing the sweet spot is probably 750-850W. You can get decent well built gold PSUs in that range dirt cheap, running a 750W PSU at 550W (DC) won't really get it into the bad part of it's efficiency curve, and they're still super easy to source.
1. Throw into that mix the 550W PSU being a server one - smaller (yes), less efficient than a 1100W (no)
2. If we get to the sweet spot being between 750W-850W for the S5, then I can not see how either would be more efficient than 2 550W PSU's, assuming same product line.
3. Then there will be price, I know you can pick up a multi rail 750W PSU for around GBP 50.00 (could be cheaper depending), but you can also pick up a single rail 550W server PSU for GBP 10.00
I think it is both cheaper and more efficient to run an S5 that way .... I am just not sure how the draw at the wall would differ (and that's why I put it out here, been tumbling about in my head for a while now!)
1. Not sure, it depends on the server PSU. There's a lot of 550W PSUs on eBay that are pulls from the mid-2000s, and they often aren't necessarily any more efficient than an ATX PSU.
2. They wouldn't be more efficient, but they wouldn't be much less efficient. I saw the sweet spot because you then get away from having to use two PSUs with the corresponding extra volume and cable management, and the price per watt is still good. Once you go up over 1000W, the price per watt goes up dramatically.
For example, you can go get a Rosewill Capstone 750W gold supply at Newegg for $90 shipped in quantity, no rebates or other crap. It'll still do 90% efficiency at 550W (DC) output. You could pick up two 550W supplies and be right in the 50% part of the curve, but any two supplies you get for $90 will probably do worse than the 750W supply anyway since you'd really have to shop around or dick around with buying one at a time and using MIRs to get anything other than a Bronze PSU under $50.
3. Server PSUs are a who other matter. I wouldn't even bother with a 550W server PSU, since you can find 800W server PSUs for that price just as easily. You just need to be willing to do some extra work to get them working.
