UPS sizing and active PFC: what am I missing?

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Tim James

Limp Gawd
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Feb 27, 2007
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I've read that active PFC can help by allowing a less expensive UPS since most UPSs are priced based on VA rating. However, they also have a maximum wattage rating. I've read that most companies specify the maximum wattage rating as 60% of the VA rating. My question is: do they do this because most PSUs are inefficient and they want to be conservative, or is there really some sort of hardwired limit set to 60% of the VA rating?

For example, if I went with an Enhance ENP-5150GH (http://www.80plus.org/manu/psu/documents/EnhanceElectronics_ENP-5150GH_80PLUS_Report.pdf), it draws 603VA apparent power at maximum load. Say I went and got a 650VA unit from APC. It is speced at 390W. According to that 80plus.org document, the Enhance will pull 486W real power due to its high efficiency and active power factor correction. Does that mean the APC unit is out, or is the 390W designation just an extra number they throw in so the average consumer doesn't get confused and try to run to much on there with his crappy, inefficient power supply?
 
http://en.wikipedia.org/wiki/Uninterruptible_power_supply#Choosing_a_UPS

"Besides choosing a UPS design, there are 2 key ratings to be aware of when choosing a UPS unit. The first is the volt amp (VA) and power (watt) ratings. Both the ratings represent the maximum amount of load that the UPS can support and the connected load typically should not exceed 80 % of either."

So for 486W of draw (plus monitor power draw) you need a 608W UPS + enough to power the monitor.

However I doubt your computer will be drawing 100% of that power all the time (or even most of the time)
 
I've done the basics, but must have missed the answer to my question: are those wattage specs on UPSs just derived arbitrarily assuming a 60% (or whatever) efficiency from the apparent power? Or do they determine it the other way around by specifying the UPS on what it can actually deliver in real power? If the latter is the case, I don't understand why people say that good power factors and efficiency can reduce the need for expensive UPSs because your real power needs are still high and those ratings (in Watts) rise linearly with apparent power rating (in VA) on most consumer UPSs I've seen.

Looking for more information on the interplay between those two ratings than a Wikipedia or PC Reference webpage can give I think.
 
Forget VA & go with watts, that is measurement of real power.

As you will never run your PSU at 100% load, a UPS within the "range" should be fine.
 
Tim James said:
I've done the basics, but must have missed the answer to my question: are those wattage specs on UPSs just derived arbitrarily assuming a 60% (or whatever) efficiency from the apparent power? Or do they determine it the other way around by specifying the UPS on what it can actually deliver in real power? If the latter is the case, I don't understand why people say that good power factors and efficiency can reduce the need for expensive UPSs because your real power needs are still high and those ratings (in Watts) rise linearly with apparent power rating (in VA) on most consumer UPSs I've seen.

Looking for more information on the interplay between those two ratings than a Wikipedia or PC Reference webpage can give I think.
Click to expand...

Every MFG is playing games with their ratings. I have several UPSs at work by different MFGs that have different rating for run time and wattage but they use identical batteries. Unless one is drastically mis designed, they should have similar rating.

I know that I am using a 1100VA ~660W belkin UPS at home that has worked great for me. However my roommate used an older belkin with higher ratings (1200va) that was a pos and would get overloaded by his pc. And his PC measured via ammeter had a lower power draw than mine..

So there is no clear answer for your question.
 
I think I will go to Best Buy, start with the cheapest model, and try to break them. :rolleyes: I can't stand it when engineering leans too far toward black art.
 
If you are able & willing to make the investment, the APC Smart UPS's are GREAT!

I have NEVER had a failure with 100's installed! :D
 
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