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So, I 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!

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What ever numbers you end up going with, remember the 80% rule. Your max load should not be more than 80% of your total available power source. – Skaughty Mar 30 '10 at 2:37
If there is any reasonable chance that other devices may be added to this UPS in the future then that 3KVA unit might not be as much overkill as you think. You also haven't considered the more important item, which is run-time. Run a UPS too close to its capacity and the run-time might be as little as a couple of minutes. – John Gardeniers Mar 30 '10 at 9:50
Also remember that the UPS capacity will affect how long the servers will stay up for, either to give the backup power time to come online or to give you time to shutdown the systems. – Sam Mar 30 '10 at 10:39
Thanks John and Sam. I thought about the run-time and ai only need enough time to shut down the server is the power is out for more than a few minutes. It's not critical that the server be up continuously. I'm in a research lab, not a business or data center. Also, a neighboring lab has a similar system but with 16 drives on a 1500W UPS and they did a run-time test without power and their server remained on for 30 minutes before it initiated the shutdown routine. So, it would seem that a 15500W or 1650W UPS will be plenty. – captainentropy Mar 30 '10 at 23:57

Disclaimer: I am NOT a power engineer. 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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Thanks for the excellent reply. The RAM is DDR3, 1333MHz, ECC/REG. I am not sure if they are FB-DIMMs. My itemized quote doesn't specify that. But I will assume so then. I'm glad you agree the 3000W UPS is overkill. – captainentropy Mar 30 '10 at 1:31

You can use a calculator like the APC UPS Selector to figure out what kind of UPS you need. I would use the estimated maximum draw of all the components and add a bit of a safety margin.

Note that the power capacity of the UPS doesn't really have much to do with how long your uptime is, that's determined by the size of the battery. A 2200 VA UPS will only be able to provide power to a limited number of machines no matter what you do, but the runtime can be extended through external batteries. Of course, the runtime will also depend on the load.

A 1500 VA UPS should be more than enough to keep this system up, but the exact model you purchase will depend on how large of a better you need given your desired runtime.

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I had tried the APC calculator and got 1100VA or 770 watts. But this is a custom build so I didn't know what to choose in the server pulldown list. I'm not sure what the defaults are. Same for the storage array. Either way they aren't too far off of what Nic was estimating. You too have been very helpful. I was thinking a 1500W UPS would be about right. I just asked what another group was using and theirs was a 1500W for about the same type of system. I will add more hard drives later so I will account for them too, so maybe a 1650W UPS? Thanks! – captainentropy Mar 30 '10 at 1:48

This is from the real world: One of our racks runs 10x1U Dell PowerEdge and 7x2U Dell PowerEdge (and a few other bits and bobs) off 2 x 3000VA APC UPS. Most of these servers have redundant PSUs, so one PSU is connected to one UPS and the other PSU to the other UPS (so that we can repair UPS units without powering down the rack). One UPS unrs at around 70% capacity, the other at 60% capacity. This should give you an idea of where realistic figures actually are.

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If each UPS is running > 50%, then when one is powered down you'll running at about 130%. That can't be too good. And what kind of draw do you see when all those machines start up simultaneously? – Nic Mar 30 '10 at 15:12
We have had several power outages over the last year, and all servers powered up OK. The trick is never to power down a UPS completely. They are all monitored via SNMP and we do regular maintenance checks. In the case of having to swap one out, we first provide the new UPS and then transfer individual server power connections one by one. – wolfgangsz Mar 31 '10 at 16:50

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