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How to size a UPS

Posted on 2011-02-25
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Last Modified: 2012-06-21
Hello,

I am thinking about buying me a UPS for my lab at home. There is one central server which I want to stay up even in the event of a power blurb. But am not sure how to size the UPS.

I am thinking about buying something like this:
http://www.newegg.com/Product/Product.aspx?Item=42-102-089&SortField=0&SummaryType=0&PageSize=10&SelectedRating=-1&VideoOnlyMark=False&IsFeedbackTab=true#scrollFullInfo

The machine has a 450 Watt power supply. The UPS listed above has 900 Watt output. Does that mean I could connect my machine and an additional one as long the second machines power supply is not bigger then 450 Watt? Do computer power supply pull the complete Watt number when they run?

Sorry I know those are two questions but they kind of belong together.

Thank you
Mc2102
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Question by:Mc2102
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6 Comments
 
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Expert Comment

by:4runnerfun
ID: 34981095
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Expert Comment

by:4runnerfun
ID: 34981111
The power supplies will pull the power as needed. The more taxiing processes you're running, the more power it'll pull.
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Martin_J_Parker earned 1336 total points
ID: 34981421
You might find it useful to buy a power meter to see just how much power your PC is actually pulling in real usage.  Something like this: http://www.powermeterstore.com/p4578/killawatt_ez.php
PCs often don't use the full available voltage, but it could be getting close if you've maxed out drives, graphics and RAM.

If you are pulling close to the full 450W then yes, you could still add another 450W PC - but the specs on that UPS say that you get 18 minutes at full power, but only 6 minutes at full power, so the question then becomes is that long enough for the level of protection you want?

You can't load that UPS above its 900W rating, so a power meter check before adding additional devices would be a good idea.  

Don't forget that your 450W PSU doesn't include the monitor's power requirements - or DSL box, switch, or anything else that you might want to remain live once the power goes out!
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Assisted Solution

by:Martin_J_Parker
Martin_J_Parker earned 1336 total points
ID: 34981443
I meant watts, not voltage!
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Assisted Solution

by:aleghart
aleghart earned 664 total points
ID: 34982659
450W is tough to pull on a normal desktop.  You need to load up with high-end video cards and a bunch of drives.

But, do the math...if you put 900W load on a 900W-rated UPS, you'll be lucky to get 2-3 minutes of time before shutdown.  As the battery pack gets older, this time will do down until eventually your load exceeds the capacity of the battery.  So, a power dip = instant power off.

I run my UPS at less than 50% load.  During startup, when all the fans spin up at full speed, the load increases, then goes back down to "normal" running load.  If you lose power, you lose air conditioning...which means that the fans will kick into high gear again.

If all you're looking for is an orderly shutdown (with software installed on both computers), then two desktops (and no monitors) should be fine on a 900W UPS.  But, I don't know of any that come with a network card or allow multiple computers to connect simultaneously for the monitoring software.

Your second computer (not attached via serial or USB cable) will not get the shutdown signal, and will get a hard power off when the battery dies.
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Author Closing Comment

by:Mc2102
ID: 34983794
Thank you. That was the information I was looking for.
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