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Thread: What PSU do I really need?

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    What PSU do I really need?

    I have been doing some research on the matter but remain unsure. This commenter on Tom's Hardware states the following:

    Quote Originally Posted by geekapproved
    Calculators often overstate your needs, obviously for their own protection. The main users are CPU and GPU, the rest is minimal and almost always under 100w. So if you have a 65w cpu at stock speeds and a HD7850 (110w) at stock speeds you can guarantee that your system is going to use less than 275w and a 380w psu would be more than sufficient.
    Going by that, I would have 250w for the GPU at maximum power consumption, 84w for the CPU, 32w for the fans, and 100w for everything else. That's 466w. If I ever go CrossFire, then that's 250w + 250w + 84w + 32w + 100w which equates to 716w. Surely an 800w PSU should asuffice, correct?

    Now, putting all the available data into this PSU Wattage Calculator, I get a minimum PSU wattage of 641w and a recommended PSU wattage of 691w. Adding in an additional GPU pushes those numbers up to 870w and 920w respectively.

    Yea, that aforementioned commenter did say that these calculators overstate the needs of the user, but by that much? One source tells me I can go for an 800w PSU, the other says I should go for a 1000w PSU. Who should I believe?
    Last edited by Zeven; 04-02-2014 at 09:27 AM.

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    Re: What PSU do I really need?

    It's a lot easier if you just check some reviews. Your estimate for a crossfire rig is actually more in line with a quad-GPU rig, i.e. http://hexus.net/tech/reviews/system...on-sli/?page=5

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    Re: What PSU do I really need?

    Quote Originally Posted by jim View Post
    It's a lot easier if you just check some reviews. Your estimate for a crossfire rig is actually more in line with a quad-GPU rig, i.e. http://hexus.net/tech/reviews/system...on-sli/?page=5
    What on Earth? 2 x EVGA GeForce GTX 690 4GB in SLI and only 739w? I should be quite alright with an 800w PSU then. Is this another case of specifications being useless and only benchmarks being trustworthy?

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    Re: What PSU do I really need?

    quad-GPU system pulled 739 watts from the wall
    You can factor in efficiency too!

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    Re: What PSU do I really need?

    Quote Originally Posted by Domestic_Ginger View Post
    You can factor in efficiency too!
    The power supplies I am looking at have 80 Plus Gold and 80 Plus Platinum efficiency respectively.

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    Re: What PSU do I really need?

    Quote Originally Posted by Zeven View Post
    The power supplies I am looking at have 80 Plus Gold and 80 Plus Platinum efficiency respectively.
    I think he means that you need to take the words 'at the wall' quite seriously when looking at reviewed draws If a power supply is 86% efficient and draws a peak of 739W at the wall then the computer is only drawing ~635W so you would be OK with a 650W rated supply. For a quad GPU system, with a hexa core power hungry CPU.

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    Re: What PSU do I really need?

    What I'm saying is that drawing 739w from the wall and efficiency of say 90% for your PSU; 739 x 0.9 = 665W is actually being supplied by the PSU.

    EDIT: Drat! ninja'd

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    Re: What PSU do I really need?

    Yep pretty much, although there's a problem with your numbers.

    The big problem is "how much wattage the cpu uses?" this number is very very hard to know, your best bet is via maths.
    There is the quick and dirty Frequency*Voltage^2

    Both AMD and Intel list cpu TDP in watts, but TDP is not electrical wattage, it's the maximum required heat dissipation of the cpu cooler under "normal" worst case continuous load and still remain in operating temperature.
    "normal" because they run tests based off what they deem as "normal" usage, running a synthetic benchmark will push a cpu more than "normal" worst case load

    Now electrical wattage will always be higher than TDP but the actual TDP isn't always the same as the listed TDP.

    Similar situation for PSU, the listed wattage isn't always the actual wattage, also you need to look at the amp's at different voltages, not just the total wattage.
    That's one reason most of the PSU calculators over estimate the required wattage, how much by depends on the calculator.

    Next you have the factor that most people over watt the PSU for "wear", as you increase the load on a psu you increase the "wear" on the capacitors and transistors, thus reducing the life span of the PSU.

    Finally you have the factor of the psychology of people, we have a general response that higher numbers = better, so we tend to over spec.

    So yes your system could probably be powered by a good 400w psu, could you run a pair of GTX 570 on your current 650w psu, probably.

    The best advice I'd give you is to get a wall socket power meter, plug it in, run a stress test on the cpu at the same time as running a graphics card stress test and see what your pc is pulling from the wall and work from there.

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    Re: What PSU do I really need?

    Quote Originally Posted by Pob255 View Post
    Yep pretty much, although there's a problem with your numbers.

    The big problem is "how much wattage the cpu uses?" this number is very very hard to know, your best bet is via maths.
    There is the quick and dirty Frequency*Voltage^2

    Both AMD and Intel list cpu TDP in watts, but TDP is not electrical wattage, it's the maximum required heat dissipation of the cpu cooler under "normal" worst case continuous load and still remain in operating temperature.
    "normal" because they run tests based off what they deem as "normal" usage, running a synthetic benchmark will push a cpu more than "normal" worst case load

    Now electrical wattage will always be higher than TDP but the actual TDP isn't always the same as the listed TDP.

    Similar situation for PSU, the listed wattage isn't always the actual wattage, also you need to look at the amp's at different voltages, not just the total wattage.
    That's one reason most of the PSU calculators over estimate the required wattage, how much by depends on the calculator.

    Next you have the factor that most people over watt the PSU for "wear", as you increase the load on a psu you increase the "wear" on the capacitors and transistors, thus reducing the life span of the PSU.

    Finally you have the factor of the psychology of people, we have a general response that higher numbers = better, so we tend to over spec.

    So yes your system could probably be powered by a good 400w psu, could you run a pair of GTX 570 on your current 650w psu, probably.

    The best advice I'd give you is to get a wall socket power meter, plug it in, run a stress test on the cpu at the same time as running a graphics card stress test and see what your pc is pulling from the wall and work from there.
    and there's also the whole efficiency curve vs power draw to worry about. You generally want to be between 30%-80% load to get best efficiency, and a fair number of PSUs have poor efficiency below 10% load. Which makes you think about idle state vs flat out running. Too large a PSU for no reason could see you running at 5% idle and 30% only when flat out. 650w - 750w is fine. 500w for single GPU system and not too many PCI cards, drives etc.

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