UPS power requirements for server

Just placed an order for a new server. The company recommended that I get a 3000W UPS. (!) As best as I could I calculated the following wattage consumption based on benchmarked data or datasheets provided by the manufacturers of each component:

                  number        watts     **total watts**
MoBo                  1           240        **240**
CPUs (E5540)          2            80        **160**
RAID cards (3ware)    2            18         **36**
RAM (6x4GB)           6             3         **18**
DVD drive             1             7          **7**
floppy                1             2          **2**
RE4 drives            8             7         **56**
WD20 drives           8             6         **48**
Intel X25 SSD         2             0.15       **0.3**
                                     total = **567**

So that is for the PSU requirements only. The PSUs in the machine are a 720W for the master node and 800W each for two subsystems. That’s a total of 2320W that can be delivered by these PSUs. But that is 4X the amount being consumed, at most, by the components. I didn’t count case fans or the eSATA card (3W maybe?) or what the PSUs themselves require but assuming I double or triple my calculations I’m not even remotely close to the 3000W UPS I was suggested to get. They run at least $1100. I could get a 2000W for about $750 or a 1500W for $450 and still be well over my estimated power need. I don’t think I need a whole lot of run time in the case of a power outage, maybe 20 minutes max, enough time to shutdown if the power doesn’t come on within 5-10 minutes.

Any thoughts? Am I off on my calculations? Did I overlook something major? If so what are your suggestions for a UPS? Thanks!

Best Answer

Disclaimer: Take the following as a set of suggestions, not as gospel answer for your particular situation.

Power factor
It’s important to be aware of the difference between Watts (W) and Volt-Amps (VA). In a regular DC circuit, it’s true that W = VA. But the process of converting sine-wave AC power to straight DC power isn’t perfect, so some of the current is wasted. This is called power factor, and is different from the efficiency measurement.

Computer power consumption is typically measured in watts, whereas UPS devices are measured in terms of the volt-amps it can deliver. A power supply with high power factor (0.99 or so) will run much longer on a UPS than a PSU with lower pf (0.6) would. Most good power supplies tend towards a power factor 0.99 so it shouldn’t be a huge issue, but you do want to be aware of it.

Spin up
Harddrives consume a lot more power when they are spinning up than when they are just working or idling. This can go as high as 25W. If you aren’t using some kind of staggered spin-up system, you should be allocating closer to 400W for the drives, just so you can handle startup.

Is the ram registered or fully buffered? If so, it’s going to consume more power than a standard DIMM. An FB-DIMM consumes something like 10W, which triples your memory estimate.

Don’t discount fans too much. I have a few Scythe Ultra Kaze which each draw as much as a harddrive. Obviously, this matters more if you have a lot of fans.

These modifications put us somewhere around 900W. Assuming your power supplies are properly sized, you can expect to get around 80% efficiency, for about 1100W. Add in power factor and the VA is a little bit higher than that. Since you want to have at least 25% buffer, a 1500VA UPS would probably be just sufficient for your needs. A 2000VA would be a more comfortable solution. 3000VA might be overkill.

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