Does a server that has dual redundant power supplies place double strain on a UPS? The reason I ask is because I am looking to purchase a UPS for my server rack, and I need to know the exact overall wattage that I need to cover. I wasn't sure if a server that is running dual 500 watt power supplies should be configured as 1000 watts when puchasing the server. Do both power supplies place strain on the server, or does the server only use one power supply, then automatically switch (on the fly) to the second power supplu

Just using the word "sever" usually conjures of visions of "we need reliability", "our stuff is important", and "has to operate no matter what."

Therefore, I wouldn't scrimp, and I'd say that at LEAST 100% overengineering would not be out of line.

Don't forget that it's not just the wattage rating of the inverter---it's how large the battery is, and therefore, HOW LONG the thing will supply rated wattage.

I was envolved, for a period, installing equipment into places that we won't go into here, but these outfits HAD HAD to operate 24/7, and even if they DID go down, had ways to shed operations responsibility over to another location. Even though these particular, uh, entities had autostart generators, they STILL put in UPS units that could power them for hours on end.

I guess it's just how important your operation is, and how important the data is.

If you want the redundancy function then you should have separate UPS's, one for each power supply and each circuit should be on it's own separate power lines, with separate breakers.

right on wizard-fred! thats my setup no sense on having servers go down just for a bad breaker.

Golly gee wilerkers everyone! Learn to Internet Search

OK, since nobody has bothered to answer the actual question that Chris asked, I suppose I will. Let me first make it clear that I am no expert in this area, and therefore could be wrong, but if so hopefully at least I will get people on the right track in answering the question. According to Silent PC Review, "the nature of a switching power supply is that it delivers as much power as is demanded by the components. This means that when installed in a PC whose components require 200W, a 400W PSU and a 250W PSU will each deliver 200W." (http://www.silentpcreview.com/article28-page3.html) Essentially, a PSU will only provide the power output that the components are drawing on it, which to me means that if you have a redundant PSU, since by definition it does not have a load on it, it will not be drawing much current itself, and any small amount it would draw would likely be negligible compared to the draw from the primary PSU, meaning it would not have much impact on the choice of a UPS. As the others have pointed out, in the case of a true, full-blown server, it might be best to consider putting each PSU on its own UPS. However, if you, like me, just have a personal server in which uptime is important, but not a necessity, and you don't want to spend the money on a second UPS, then I would say you should be alright basing the rating of the UPS off of a single PSU. As I said, this could be incorrect, so if somebody has better, more accurate knowledge on the matter, please let me know.

Verigo: What you say is true, the UPS only has to supply the consumption of the computer at normal load. Having redundant power supplies would only require a little more power to keep the other at idle. However there is a BIG exception. Unless you have special turnon circuits, when you first turn the computer on the startup surge would be double since the capacitors in each power supply would be simultaneously charging.

True, however, whether or not this would matter would depend on the type of UPS being used. If it's a typical "low-cost" UPS of the off-line type, then it only runs off the battery during a power outage. As long as AC power is available, the battery isn't even connected to the computer--it is as if the computer is connected directly to the wall. So unless you're turning the computer on during a power outage, which makes absolutely no sense, it won't be a problem. On-line UPS's are the ones in which this power-on current spike would present a problem, but chances are, anyone spending the money for an on-line UPS is going to get one with enough current supplying capacity to handle this.