Introduction

Anyone building a computer system should eventually pose the question: How much power does the system actually require? This is an important consideration, since it's impossible to choose an appropriate power supply without actually knowing the demands of your system. Unfortunately, many users take the easy way out: just grab a 500W power supply and call it good. If you really want to be safe, you can even grab on 800W PSU... or if you plan to run multiple graphics cards perhaps you really need a 1000W unit, right?

If people really took the time to examine system power requirements, we would see a tremendous increase in sales of 300W to 400W PSUs. The truth is that the vast majority of systems would run optimally with such a "small" power supply. Even if you're running SLI/CrossFire, you don't actually need a 750W power supply. (Of course, we recommend purchasing a good quality power supply, as there are certainly "750W" PSUs out there that can't reliably deliver anywhere near that much power.) To help dispel some myths relating to power requirements, we've put together a couple of charts.

GPU Power Consumption*
Manufacturer Idle Load
NVIDIA GeForce 9600 GT 49W 107W
NVIDIA GeForce 8800 GT 64W 115W
NVIDIA GeForce 9800 GTX 79W 116W
NVIDIA GeForce 9800 GX2 90W 179W
NVIDIA GeForce 8800 Ultra 100W 186W
ATI Radeon HD 3650 17W 32W
ATI Radeon HD 3850 53W 82W
ATI Radeon HD 3870 62W 92W
ATI Radeon HD 2900 XT 67W 104W
ATI Radeon HD 3870X2 55W 130W

* Actual power consumption for the graphics cards only. Results taken at idle on the Windows desktop and under full load running the Fur benchmark.

CPU Power Consumption**
Manufacturer Idle (EIST or CnQ Enabled) Idle Load
Intel Core 2 Duo E4500 14W 17W 36W
Intel Core 2 Duo E8500 18W 22W 43W
Intel Core 2 Quad Q9550 19W 23W 60W
Intel Core 2 Extreme QX6850 29W 32W 103W
Intel Core 2 Extreme QX9770 26W 56W 86W
AMD Athlon 64 X2 5000+ 33W 47W 89W
AMD Athlon 64 X2 6000+ 25W 74W 160W
AMD Phenom X3 8750 50W 67W 86W
AMD Phenom X4 9600 BE 29W 36W 101W
AMD Phenom X4 9850 BE 38W 53W 126W

** Actual power consumption for just the processor. Results taken at idle on the Windows desktop with either EIST/C&Q enabled or disabled, and full load generated using BOINC.

Chipset/Motherboard Power Consumption***
Platform and Chipset Load
Intel P35 (775) 37W
Intel P965 (775) 39W
Intel X38 (775) 52W
Intel X48 (775) 40W
NVIDIA 680i (775) 46W
NVIDIA 790i (775) 51W
NVIDIA 750i (775) 59W
NVIDIA 780i (775) 69W
NVIDIA 8200 (775) 29W
AMD 690G (AM2) 34W
AMD X3200 (AM2) 35W
AMD 770 (AM2) 40W
NVIDIA 570 (AM2) 40W
AMD 790FX (AM2) 42W
AMD 790X (AM2) 43W

*** Actual power consumption for the motherboard and chipset. Idle and load power do not differ by any significant amount.

Top-end graphics cards are clearly one of the most demanding components when it comes to power requirements in today's systems. Only heavily overclocked CPUs even come close to the same wattages. Note that the above chart only includes last generation cards; NVIDIA's latest GTX 280 requires even more power.

Looking at the processor side of the equation, Intel's Core 2 Duo/Quad/Extreme CPUs in general have very low power requirements. AMD's latest Phenom processors aren't far behind, however, especially in light of the fact that they include the memory controller rather than delegating the task to the chipset. We should also mention that part of the reason for the extreme power requirements on the X2 6000+ come from the use of an older 90nm process.

Naturally, motherboards also require a fair amount of power. Current motherboards average around 47W for socket 775 and 39W for socket AM2/AM2+, but features and other factors can heavily influence that number. Outside of their IGP solution, NVIDIA's chipsets tend to use more power than the competition; AMD chipsets on the other hand typically require less power. Again, numerous other aspects of any particular motherboard will impact the actual power requirements, including BIOS tuning options.

Hard drives and optical drives account for another 10 to 20W each. However, remember that hard drives are a relatively constant 10 to 15W of power draw (average is around 12W) since the platters are always spinning (i.e. idle), and movement of the drive heads during read/write operations (i.e. load) only increases power draw slightly. Optical drives on the other hand stop spinning when idle, requiring only about 5W, while during read or write operations they need around 18W.

RAM power requirements measured a constant 2W per DIMM, regardless of capacity (though clearly not including FB-DIMMs). That figure is estimated, unfortunately, as we could not measure DIMM power requirements directly; we measured power draw with two DIMMs and then again with four DIMMs to arrive at the reported figures. It's also not possible to easily separate memory power requirements from the motherboard and chipset, as they share many of the same power connections from the PSU.

Building Three Sample Systems
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  • Dancer - Thursday, October 22, 2009 - link

    On the face of it, this article seems an excellent, well-researched contribution to a highly vexed question. I do have a concern, however: We know that the power output of a PSU drops as it ages. We also know that this drop depends partly on quality and partly due to random chance. If I'm buying a PSU to last 3, 5 or more years, will this seriously affect the capacity of the PSU I should buy for a given machine?
    Reply
  • internetrush - Saturday, June 06, 2009 - link

    Ok, lets low ball it, im running 200w (average) per graphics card, about 50w cpu (core i7) and three hard drives.

    Lets see

    200 x 2
    +50 + 50 (motherboard chipset)
    +30 Sound card
    +10 cd drive
    +20 (fans)

    During a game, much less a stress test, im lowballing a 600w load on my PSU.

    If i had an 800w PSU that would be 80% of its total output, which thereby increases its heat and decreases its life.

    When you buy a 1000w PSU, not only are you ensuring that you will never watch your computer go up in smoke (had a friend do that to a 350w on an old P4) but you are also not having to replace it whenever you buy a new processor or add something to your system.

    This article is good, however, on a tech website i would expect a bit more consideration for the higher end gamers and common sense.

    Common sense says, if you are a higher end system user, you WILL expand said system!

    For gods sake! Some cards today use up to 500w power (the 4890X2 and new 295 SuperCard).

    As a gamer, id rather have a 2000W PSU that id never have to replace than a 400w that would FRY as soon as i threw on a new video card.
    Reply
  • Christoph Katzer - Saturday, August 08, 2009 - link

    Sorry for the late reply.

    You are probably right when you see it from the perspective of a high-end-hardcore-gamer... But do you know how small the percentage of people is who actually own a real high-end system?
    Reply
  • JohnMD1022 - Sunday, March 15, 2009 - link

    It would be nice if you could periodically update this with newer components. Reply
  • lopri - Tuesday, April 28, 2009 - link

    quote:

    quote
    Reply
  • BillyBuerger - Sunday, October 26, 2008 - link

    Anyone have any info on that Thermaltake QFan 300? That thing looks great efficiency wise. Not normally a Thermaltake fan. And the fan controller looks like it sucks. Just keep it below 150W... Or fan swap. Reply
  • Christoph Katzer - Thursday, October 30, 2008 - link

    I will have a review up soon! Reply
  • Cincybeck - Tuesday, September 30, 2008 - link

    Couple of "knowitall" friends were trying to tell me I was going to need a larger power supply when I built my new system. Which in turned incited the Microcenter sales person saying oh yea you're probably going to need that too. I turned around said I estimated these parts to draw at most around 200, 250 watts, and I have a 500W Seasonic M12. Shut him up pretty quickly, but my friends were still pushing it the whole way home. So now if they ever bring it up again I can print this article and shove it in their face. Thanks =D Reply
  • 0roo0roo - Sunday, September 28, 2008 - link

    i like the graphs:)
    keep it up!
    this is the info we need!
    normally the psu market is just lousy because of the lack of any real information.
    Reply
  • mark84 - Friday, September 26, 2008 - link

    For those quoting that old link for the AtomicMPC graphics card power thread, the new/current one is being maintained here http://forums.atomicmpc.com.au/index.php?showtopic...">http://forums.atomicmpc.com.au/index.php?showtopic... Reply

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