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UPS Power Rating Calculation Help Required

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Last response: in Components
December 19, 2009 6:18:36 AM

1. I need to decide on what UPS to buy for my system.

2. But first I need to decide how beefy it needs to be - this is where I need your help.

My SMPS is a 650W Active PFC, 80+. Details :
Brought it because it was cheap.

3. Here are my system details:

CPU : Q8400, TDP 95W. Details :
MB : GA-EP45-UD3P, Max draw 196W. Details :,,, ( this posts 242W at full load )

I don't have a clue what these consume ( any ideas? links? )

RAM : TWIN2X4096-6400C5. Details :
GPU : GeForce 9800 GT 1GB. Details :

To these I will hookup an externally powered HDD, internal SATA HDD, DVD Writer, mouse, keyboard - standard stuff.

The monitors will be externally powered, and NOT hooked to the UPS.

Taking all these things into consideration, I just pulled a number outta my a*s - 350W would be my system draw at peak load.

What do you guys think?

4. That means, the load to my SMPS would not be more than ~350W at peak load?

In that case, my UPS needs to supply this much juice:

sqrt ( 2 ) * 1.25 * 350W = 1.8 * 350VA = 630VA

( sqrt ( 2 ) -> power factor, 1.25 -> 80% efficient SMPS )

5. So I believe a 750VA UPS should be good enough?

My calculations get totally screwed if my system consumes more than 350W and this is what I want to confirm with you guys - to help me figure this out.

6. As the SMPS is APFC, I can play a game by buying a UPS without pure sinewave output - would save me a lot of bucks.
Otherwise, I can just buy myself some peace of mind by paying more than double for a pure sinewave UPS.

This is were choices become more difficult to make.

A 750VA APC costs ~$90 less than a 1KVA APC - both pure sinewave.

What you say - should I get the 750 or the 1k?

I am not sure however, whether APC has derated their UPS or is the VA rating a peak rating.

In case the 750VA is a peak rating - maybe I should, then, get the 1KVA?

This is a horrible dilemma!

More about : ups power rating calculation required

a b ) Power supply
December 19, 2009 7:48:52 AM


It's not just getting a sufficient VA rating. The batteries wear out and a UPS with a higher rating will last longer. I don't have any hard numbers and it's likely a UPS with better quality batteries could last longer than a UPS with a higher VA but in general a higher VA will last longer.

Personally I'm not happy with the UPS situation. They are too expensive. I'd like a much smaller UPS that supplies just enough power to shut down my system without it crashing and I can't find one.

FYI, I bought a Belkin 1000VA 5 years ago for a system that probably didn't use much more than 150Watts peak and it lasted three years. I bought another one for my sister who had a less powerful machine and it lasted about 2.5 years. The batteries in many UPS's are no different so don't count on much more than 3 years.
December 19, 2009 8:26:25 AM

Frankly, it seems darn stupid to me that motherboard manufacturers do not feed from a set of 12V Rails.

How much would a few regulators cost - compared to the cost of all the hoola hooping UPS and other backup methods?

Even if they had a 12V and 5V feed, it would be much more cheaper for people to hookup a battery to it and save on mighty UPS costs.

When on power - draw from the SMPS
When off mains power - draw from a 12V battery.

Would have saved $$ I tell ya.

Had it not been for this Active PFC stuff, I would have bought myself a cheap 1KVA UPS from my friend's owm company - the last 650VA unit I bought from him is still going strong since 2001, albeit a $20 battery replacement last year.
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December 23, 2009 3:31:42 AM

It seems like some SMPS ( like from Antec and Corsair ) do have a primary 12V rail off which they have two "child" switching PSU's running to get 5V and 3.3V

It would be interesting to see if these could be modded to run off 12V battery when power goes off ( the switching ciruits would need to be mod accordingly to DC signals )

Anyways, I have decided on the XL version of the Smart 750 from APC.

Should be more than sufficient I think.