Power Supplies and "real" power draw

[FunkZ]

I got an APC SmartUPS 700 to provide backup power for 2 of my machines. The SU700 is only rated for 450W maximum draw, which seems low considering I’m using a Thermaltake 420W in one PC and an XClio 450W in the other. But of course those are maximums and they are not drawing anywhere close to that.

On the UPS there are 4 out of 5 load lights lit, which according to the APC manual means it is between 67% and 85% load. The lights represent 17%, 33%, 50%, 67% and 85%.

So this means that both of my systems together running at 100% CPU load only draw between 301 and 382W. When I turn off F@H it drops down to only 3 lights, that's somewhere between 225 and 301W total for both systems.

I was just surprised because lots of folks state that you need 500W, 600W or more to power your system. Obviously this is not the case. I would agree you don’t want some cheap crappy 300W OEM power supply that came in a $20 no-name case, but I think as long as you go with a quality name brand power supply that 400W is plenty to run most systems unless you've got some major power hogs running off it. Also, with overclocking, steady/stable rails are probably more important than just having really large ones. Most power supplies usually hit their peak efficiency at around 30-50% load anyway which should mean mine are operating in that sweet spot.

BTW - don't buy one of those cheap BackUPS and try to run your system off it. I got a BackUPS 350 at Staples, it looks like a power strip on steroids. It couldn't run even one of my systems for 1 second before shutting down. So now I'm using it to provide backup power for my cable modem, router, kvm, etc. which is fine for it. The SmartUPS series is more of a business-class UPS, the SU700 is rated for 15 minutes at 50% load and 5 minutes at 100% load, which is plenty of runtime for the typically brief power dropouts we get here.

The SmartUPS 700 is a discontinued model, but you can pick up a used one with new batteries and a 1 year warranty for under $100 shipped off eBay.

[jester]

Theres one thing your not considering...

your pc psu is putting out 420w DC (direct Current) power. Your wall & the ups is on AC (alternating current).

The AC power consumption of your psu's will be different of the DC power output which is what the manufactures always give.

I dont know the formula for figuring these kind of things out anymore as Ive been out of the business since '99. Look on the psu's themselves, it might tell you what the actual AC consumption is. Might try the manufacture's site as well.

[FunkZ]

Isn't that why it's called a SmartUPS 700 but max wattage is listed at only 450W? It is figuring only about a 65% efficiency of the devices plugged in.

It has a max of 700VA (volt amps) which is how draw on AC is measured, correct? For example, 115V x 6A. But for devices you plug into the UPS that convert AC to DC, the max is 450W.

I am by no means an electrician so please tell me if I am wrong here.

But your post did make me think about draw being different and I did realize that most modern power supply efficiency is around 70-80% in most cases.

So if the UPS load indicator is registering 80% load and that comes out to a draw of 560VA of the maximum 700VA it is capable of, and the power supplies are converting 75% of that into DC, then they're putting out a total of 420W between them which is a good bit higher than what I stated earlier. But still only around 200W to each one. That's less than half of their rated output of the supplies.

[jester]

Ups's are rated at AC power consumption, unless you have like a 12v output on it.

I looked at pcpowercooling's web site (this is just an example), while they dont list the ac wattage consumption they do list how many amps it draws.

Their 510W psu puts out 34amps just on the 12v rail. It only uses 10amps of AC power. Ohms law does state that the higher voltage you go, the less power a device will use (because higher voltage doesnt have to push as hard to operate something) so the current (amps) & the wattage consumption of a 12v DC device will be higher than the same device on 110v AC.

This is what I was getting at earlier. The wattage consumption that its actually using out of your 110v wall outlet will be lower than the wattage that is getting to your computer.

hope this helps

[silenze]

The UPS that Fawkit.org is on sits at just 10% with the tower and dsl modem plugged into it... not sure what the rating is off the top of my head..

[FunkZ]

I think we're saying the same thing Jester, just in different ways.

Yup, the SU700 has a maximum draw of only ~6A at 115V (6*115=700VA) of AC.

If that PCP&C 510W power supply has a maximum draw of 10A of 115V and you had it fully loaded it would be too much for this SU700.

If the SU700 can handle 6A at 115V then at 12V it can handle 58A. (700/12=58 ) The max draw of my XClio 450W PSU is 9A at 115V or 86A at 12V so if it was pulling anywhere near max it would be too much for the UPS as well.

Considering I'm running both this power supply plus another 420W off the same UPS and it's only showing between 67 - 85% load, I'm not drawing anywhere near max of the power supplies.

Now I did forget to bring up something else, that even if my CPUs are at 100% load running F@H that the video card, hard drives and DVD burner are probably not drawing much. I suppose I would have to be running Aquamark, F@H, a defrag and burning a DVD all at the same time in order to try and max out everything in the system, and that could possibly exceed the rating of the UPS if I did that on both systems at the same time.

But my point still stands that for the average modern system a 400-450W PSU is plenty enough.