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I just bought a new UPS. Now since I started using it I experienced at least 3 random freeze in Windows 7 and it happens during the first 10 minutes of start-up. Note when I say freeze, I mean I literally can't do anything in Windows, everything is frozen including the mouse cursor.

Is it possible that I bought a broken or underpowered UPS? Can an underpowered UPS make a computer freeze?

BTW, this power calculator says I'm under 300W, but I do have 500W PSU.

Update: FEB 9, 2012

I have replaced my APC UPS with another brand: FSP UPS which is the same brand with my PSU. It's been running with no problem for the last 5 days.

In reference to my answer below: Could it be really true that non-SmartUPS APC models are not compatible with active PFC PSUs? I find it hard to believe since most branded PSUs are active PFC.

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If your components draw 300 W from a PSU with 75% efficiency, the PSU will draw 400 W from the UPS. –  Dennis Jan 28 '12 at 17:47
A UPS can expose problems in the PC power supply. If it's a cheap, no-name power supply or pushed to its limits, it may not meet the ATX specification's hold up time requirements. This usually causes reboots or shutoffs though, not freezes. –  David Schwartz Jan 28 '12 at 21:54
@david i'm having exactly that situation on a machine and its been driving me mad. I'm going to try a new psu as what you say makes sense. –  Sirex Feb 3 '12 at 10:39
So plug the PC back into the wall and bypass the UPS and see if the problem continues. If it does, it's not the UPS. If it goes away, take your UPS back to the shop and exchange it for a more powerful one or if you can't, sell it off on ebay and buy a better UPS. –  Matt H Feb 3 '12 at 10:55
@MattH You are certainly correct that if the problem happens without the UPS, it cannot be the UPS's fault. But it is not the case that if it only happens with the UPS it must be the UPS's fault. It can be the power supply's fault. (For example, if it doesn't meet the ATX specification's hold up time requirements, it can fail with a UPS but work fine without one.) –  David Schwartz Feb 3 '12 at 20:00
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3 Answers

Initial Complaint Computer freezing on startup, after connecting to UPS:

Yes, when your computer is starting up, it pulls more power initially to load up everything, which can cause an excessive drain on a UPS that isn't slated to handle the peak power consumption of your computer, hence the freezing.

UPS Recommendation and Breakdown Given that you have a 500W PSU in your system, and you probably have your computer, monitor, printer, cable modem/router/dsl modem, scanner (if you have one), and you probably have external speakers for your computer, you are looking at most likely a peak performance of 600Watts. Thus, you are probably looking for a 750Watt UPS, not the 500watt one.

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The printer,scanner, cable/dsl modem, and speakers shouldn't be on the battery backup. They need to be on the part that is protected by the surge protector, but you don't need them to work when the system is on battery backup. The purpose of the battery is to allow you to run long enough to save your work –  mhoran_psprep Jan 28 '12 at 20:23
@mhoran_psprep, given that the OP didn't state what his configuration was, I was using the most common options when doing the UPS calculations. –  zackrspv Jan 28 '12 at 20:27
If the UPS is overloaded, it shuts down. Incorrect operation of the computer should never happen; either it runs fine, or shuts down, there should be no in between. –  psusi Feb 3 '12 at 15:01
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up vote 2 down vote accepted

After returning the item to where I bought it, we are unable to figure the problem until I contacted APC support:

APC support said my PSU is incompatible with my UPS. Moreover they said, any PSU with Active PFC (Power Factor Correction) is not compatible with my UPS. So the solution is to get a PSU that is none PFC or get a compatible UPS (usually APC Smart UPS however it is 8x more expensive).

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APC support is full of it. There may be some badly behaved power supplies out there, but most quality supplies these days have PFC and work just fine with a UPS. In any case, it is only an issue while running from the battery. –  psusi Feb 3 '12 at 14:43
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When we first bought UPS items we got the cheap ones. They were just enough power, they were "unregulated" for these it meant that the power had to take a good dive before they would switch. It was not long before we realised that they had zero value. I could test them by pulling the plug , but lazy brown-outs (vrses black-outs) would still knock the computer up. After that we got overrated, and fully regulated "brand name" ones, and have not had any of the problems we had with the cheap ones. I couldnt even sell them to anyone, because of how useless they were.

That only ever occured when there was an actual brown-out, not 10 minutes into startup. It is possible that the thing is too weak. but . . Init/post, and then startup is where the largest ammount of power is needed, usually not 10 minutes in, unless you are firing up a 3D game right after startup.

Test with the UPS removed? Do you have any other high consuming things on the same curcuit to begin with? refridgerator compressor, space heater, toner type printer (fuser), air conditioner, microwave? things that would cause a minor power surging?

Did you test the computer in safe mode? or with a few startup items turned off via Msconfig, or by stopping a few items in task manager before your 10 minutes runs out? What are the temps on the video card?

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