UPS Power Rating Calculation Help Required

[Anonymous user]

1. I need to decide on what UPS to buy for my system. Of course it's going to be an APC

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 : Newegg.com - Antec EarthWatts EA650 650W Continuous Power ATX12V Ver.2.2 / EPS12V version 2.91 SLI C...
Brought it because it was cheap.

3. Here are my system details:

CPU : Q8400, TDP 95W. Details : Newegg.com - Intel Core 2 Quad Q8400 Yorkfield 2.66GHz 4MB L2 Cache LGA 775 95W Quad-Core Processor ...
MB : GA-EP45-UD3P, Max draw 196W. Details : Newegg.com - GIGABYTE GA-EP45-UD3P LGA 775 Intel P45 ATX Intel Motherboard - Intel Motherboards, Gigabyte GA-EP45-UD3P DDR2 P45 Motherboard | Gigabyte GA-EP45-UD3P,P45,Motherboard,DDR2,Gigabyte GA-..., Gigabyte GA-EP45-UD3P - Bjorn3D.com, AnandTech: Gigabyte GA-EP45-UD3P - P45 at its Finest ( this posts 242W at full load )

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

RAM : TWIN2X4096-6400C5. Details : Newegg.com - CORSAIR 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop M...
GPU : GeForce 9800 GT 1GB. Details : Newegg.com - PNY XLR8 VCG98GTEE1XEB GeForce 9800 GT 1GB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready...

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! Help!

[BillP]

First and foremost, results with PFC and pure vs. step-approximated sine waves are varying. Most units work fine, though some don't. I personally have a BR800BLK on my home computer with a PFC 600w PSU and it works perfectly. Others, in rare cases, have experienced loud & disconcerting buzzing noises from a PSU when the UPS is running on battery, randomly overloading UPS units that shutdown hard, or computers that turn off when the UPS goes to battery. Most of these problems - namely the latter two - are related to sizing and can easily be remedied.

The most important thing to note is oversizing. If using a Back-UPS (step-approximated sine wave) unit with a PFC PSU, you'll want to multiple the required wattage rating by 1.25. This is outlined in one of our kbases, available here: http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=8883

With what you've provided, looks like your max draw would be closer to 400w, and 400 * 1.25 = 500w

The BR800BLK model that I own (and recommend) can handle up to 540w, and remember that the 500w rating is based on max draw AND inrush current. This model should work just fine for you, although if you're the extra cautious or paranoid type, you can pay the extra for an SUA750 instead. It can handle 500w and you'd only need to size for about 400w at max draw since you don't need to worry about oversizing with the Smart-UPS (pure sine wave) models.

Hope that helps!

[voidstar_apc]

Hey buzz,

If I'm reading that k-note correctly, wouldn't the maximum possible in-rush power draw be 1.25 * the power supply's rated output power, or 1.25*650W = 812W?

[BillP · Accepted solution]

Yes, technically that's true, the PSU has the potential to pull max draw * 1.25 at initial power on. But that's more along the lines of thinking the PSU's full capacity (or close to it) is required to begin with. In theory, to be extra safe one "should" also size for what the PSU can pull max, though that's not necessary because none of the equipment in this computer will ever use that much power.

The kbase is basically outlining methods of guaranteeing compatibility - which would also recommend sizing based on your max size of your PSU (650w), because the potential is there - regardless of whether it'll result in oversizing or not. It's the absolute safest route possible, especially since we don't always know someone's exact configuration.

Since we know all of the internal components and how much power they draw, we can be more specific. In this regard, since he doesn't plan on using the full capacity of the PSU it should only require 1.25x what it's drawing, which was the 400w estimate.

I have a 600w PFC PSU on my BR800BLK (rated at 540w), which could technically overload the unit with the PSU alone, but because I only ever pull about 300w max, I've never had a single problem. Neither have any of the other BR800BLKs I've spread around to family and friends.

The only time I've ever really seen a (non-server) computer have an issue with a Back-UPS is for the smaller BE models and that's generally been related to transfer time issues (the BRs have a shorter transfer time than the BEs).

There's nothing wrong with being extra safe, you're just likely to end up with an oversized unit. If that's fine, go for it. Though remember that in this instance, if you plan on upgrading equipment in that tower, you may need to revisit your calculations.