Trying to understand PC power usage

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nintendoeats
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Trying to understand PC power usage

Post by nintendoeats » Tue Mar 24, 2015 5:56 pm

I've been wondering how close I was to the ceiling of my power supply, so I bought a Kill-A-Watt P3. I have a Radeon 7970 (which is known to be a big thirsty card) so I was expecting 350 W or so while running F@H, maybe a little less because folding clearly doesn't use some parts of the card (full load in-game reads much hotter than full load while folding). However, what I got is just under 200W. I tried running F@H on the CPU while sitting on the Sniper Elite 3 menu, which should be as big a system push as you can get in the real world (sans transferring between the SSDs) and it constantly reads just under 250W. Bear in mind, that includes the 15 - 20 percent loss at the PSU. The advice that I've seen for systems like mine generally pushes 500W, and while I have done some undervolting I don't think I've shaved off 150W doing so. Have I misunderstood how this works, or do I (and everybody I know with a gaming PC) have a massively overpowered PSU?
Last edited by nintendoeats on Tue Mar 24, 2015 6:39 pm, edited 1 time in total.

CA_Steve
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Re: Trying to understand PC power usage

Post by CA_Steve » Tue Mar 24, 2015 6:10 pm

All I can do is guess the rest of your system from your other posts - it's a Z68 mobo, so it implies a 95W CPU if i5 or i7. Don't know what you have for storage...let's assume an SSD and an HDD. Don't know what you have for a PSU - I hope it's Gold efficiency and you are mid-range in it's power envelope. So, that's ~90% efficiency. Given these assumptions:

Stress load is 95W + 250W for the gfx card + call it 50W for everything else. Total = 395W (DC), 439W (AC).
Heavy game load is in the 335W (DC) range, 373W (AC).

Don't know what your undervolting is...but a 0.1V undervolt on 1V amounts to a 19% reduction in power use (Power is proportional to freq * (V^2)).

Try running Prime95 + Furmark and see what you get.
1080p Gaming build: i5-4670K, Mugen 4, Asrock Z97 Anniversary, MSI GTX 1660 Gaming X, 8GB 1866 RAM, Samsung 860 Evo 500GB, Crucial MX100 256GB, WD Red 2TB, Samsung DVD burner, Fractal Define R4, Antec True Quiet 140 (2 front + rear) case fans, Seasonic SSR-550FX. 35W idle, 45-55W video streaming, 170-200W WoW, 318W stress test (Prime95 + Furmark)

nintendoeats
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Re: Trying to understand PC power usage

Post by nintendoeats » Tue Mar 24, 2015 6:19 pm

CA_Steve wrote: Try running Prime95 + Furmark and see what you get.
Pretty close. i5 2500k, 2 SSDs (mass storage is now in a network drive) and an 80+ but not gold 600W PSU.

The calculation you did is more what I was expecting, but if undervolting makes that much of a difference that goes some way to explaining things. My CPU is undervolted by 130mv and my RAM is at 1.35 (PCH and VCCIO are reduced a bit as well). The GPU is down about .05V, but it is also overclocked about 150 mhz.

I will do this. I am also curious. I shall report back.
Last edited by nintendoeats on Tue Mar 24, 2015 7:01 pm, edited 1 time in total.

nintendoeats
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Re: Trying to understand PC power usage

Post by nintendoeats » Tue Mar 24, 2015 6:59 pm

Peak of 325 with Prime95 and Furmark running, CPU and GPU confirmed at maximum.

You list wattages both in AC and DC. I'm assuming that PSU ratings are in DC, while the rating I'm getting at the wall is obviously AC. Is that correct?

CA_Steve
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Re: Trying to understand PC power usage

Post by CA_Steve » Tue Mar 24, 2015 7:33 pm

nintendoeats wrote:You list wattages both in AC and DC. I'm assuming that PSU ratings are in DC, while the rating I'm getting at the wall is obviously AC. Is that correct?
Yep.

325W is still plenty less than my estimated 440W (and 90% PSU). :)

Anyway, in sort of an answer to your original post...there's a couple of ways to size a PSU. I like to grab the stress load numbers as above, derate by 15% for a heavy gaming load, and then look for a PSU that's damn quiet at that load.
1080p Gaming build: i5-4670K, Mugen 4, Asrock Z97 Anniversary, MSI GTX 1660 Gaming X, 8GB 1866 RAM, Samsung 860 Evo 500GB, Crucial MX100 256GB, WD Red 2TB, Samsung DVD burner, Fractal Define R4, Antec True Quiet 140 (2 front + rear) case fans, Seasonic SSR-550FX. 35W idle, 45-55W video streaming, 170-200W WoW, 318W stress test (Prime95 + Furmark)

nintendoeats
Posts: 59
Joined: Sun Feb 22, 2015 11:57 am

Re: Trying to understand PC power usage

Post by nintendoeats » Wed Mar 25, 2015 6:10 am

Yes, fortunately mine is perfectly silent (CM Silent Pro 600w). I was actually thinking more about jamming a second card in there for folding. It would probably be too loud, but I now know that I have scads of wattage headroom. And probably won't need a new PSU until this one dies.

Thanks!

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