Building Three Sample Systems

Okay, so far we have some basic power guidelines in place. Let's put these figures into practice and look at some actual system power requirements. We've selected components for three different systems, so let's examine how much power each one requires.

System 1:
Intel Core 2 Duo E4500, 4GB Memory, P35 chipset motherboard, ATI Radeon HD 3650, an optical drive, and one hard drive. Outside of perhaps the memory, this is representative of your modern entry-level computer system. At idle, this computer requires around 90W of power. Even when we put the pedal to the metal and put a full load on the graphics card, processor, and optical drive, we still have a total power consumption of only 140W.

System 2:
AMD Phenom X4 9850 BE, 4GB Memory, AMD 790X Chipset, ATI Radeon HD 3870X2, an optical drive, and two hard drives. Our midrange system roughly doubles our power requirements, and depending on the benchmark it will offer more than twice the performance of our entry-level machine. At idle with Cool & Quiet enabled, this system uses almost 168W of power, while it needs at most 341W when fully loaded.

System 3:
Intel Core 2 Extreme QX6850, 4GB Memory, NVIDIA 780i Chipset, NVIDIA GeForce 8800 Ultra SLI, an optical drive, and four hard drives. For our third example, we chose some of the most demanding products for testing. In particular, the 780i Chipset from NVIDIA has the highest power consumption of all chipsets we've tested so far, drawing a constant 69W. (There is of course some variation in power consumption even from chips of the same family, and the features and extra chips on each motherboard differ from manufacturer to manufacturer. Our particular 780i is an EVGA motherboard.) The idle power consumption for this setup is around 310W, and once we place of full load on everything power consumption increases to 544W.

Worth mention is that the second graphics card in an SLI/CrossFire setup never actually uses 100% of the theoretical maximum power consumption. We estimate power consumption based on the figures on page one, and the second GPU only runs at around 50% power at the desktop (i.e. half the idle power draw); adding a third GPU would result in an even lower load, since the third card is frequently underutilized. Likewise getting a full load on quad-core CPUs and multiple GPUs is not a typical scenario. It may be possible to draw slightly more power, but the above guidelines should suffice.

Do these numbers help clarify the situation? The first system has very low demands, and yet if we look at the PC market as a whole 90% of current shipping systems don't even provide the same level of hardware as system one. Even with that fact accepted, the question remains: what sort of power supply should you choose for such a system?

That's the next topic of discussion, and we want to show some simple ways to help you choose the correct power supply for your needs. For the moment will put aside other important factors like DC output stability, ripple and noise, and overall quality and focus on choosing an appropriate power supply. Key factors in this decision will be the efficiency curves and noise levels.

Index Efficiency Explained


View All Comments

  • LTG - Monday, September 22, 2008 - link

    You can't just look at the TDP's, that's the problem.

    When you overclock the power demands can increase in a very non-linear way.

    So for example I can't plan for one of the most popular 280 cards like the EVGA FTW 280 GTX. It's 11% core over clocked, what does that mean at the outlet? No good way to guess.

    Same for a 3.6Ghz QC CPU - this is a very common overclock, yet there is no direct way to know it's power requirements.

    I'm just saying this is not esoteric information, this would be data people really could use and can't get from the manufacturer.

  • xaris106 - Friday, November 7, 2008 - link

    But you can. All you need is stock power consumption at load(Pstock), stock voltage(Vstock) and stock frequency(Fstock) The oc power is then:
    Poc = Pstock * (Foc/Fstock) * (Voc/Vstock)^2
  • nubie - Monday, September 22, 2008 - link

    Unless they edited this, you are operating on a false assumption.

    (Unfortunately, our power supply testing labs didn't have the latest GPUs available for testing.)

    A power supply testing lab doesn't need to have every component on hand because it uses a test bench to load the supplies.

    Great article, way to dispel myths, I guess since I only plan to overclock with a single video card and one or two hard drives my PCPower Silencer 470 will be enough power for many years to come (which is what I hoped when I bought it, the only downside is the single 6-pin for the video card, when it can clearly handle much more.)
  • JarredWalton - Monday, September 22, 2008 - link

    Just for the curious, AnandTech staff is scattered far and wide around the globe (well, at least the US and Europe). I'm west coast, Wes is east coast, Anand and Derek are in NC, Gary is in TX, and we have Johan and Liz in Belgium with Christoph in France. (That's not everyone, but you get the point.) Since we tend to focus on our own areas of testing, Derek and Anand have the most CPU/GPU hardware, I have laptops and displays, Gary has motherboards, etc. I can definitely say that Christoph isn't the only one without 48x0 and GTX 2x0 hardware. [Pardon me while I go cry in a corner now....] Reply
  • hyvonen - Wednesday, June 3, 2009 - link

    Oh, so in order to get this power draw info on more components, I should beg Anand? :) Reply
  • LTG - Monday, September 22, 2008 - link

    Totally understood, many companies now days are distributed and can't have every physical resource available to every person.

    However I would volunteer to send Christof a new 280 GTX to test if he decides it's worth it.

    Serious - Just please send it back whenever finished :). And I waive all claims if it is accidentally fried by that fancy Chroma thingy.

  • ineedaname - Tuesday, November 2, 2010 - link

    This article is well written and tried to put real life numbers and situations to the test on PSUs.

    However i felt that they should mention one other thing for people who are novice to computers. They should mention that just because a PSU is rated for 500w it doesn't necessarily mean that it will do the job even if your computer will only suck about 150w max. Because a 500w psu that comes along with a $50 case just won't do the job. Not because of the wattage rating but because they use crappy parts and workmanship; it'll just die in 3 months when the warranty is over.
  • gsuburban - Saturday, November 11, 2017 - link

    This article was written in 2008. It would be great if it were updated with the more modern CPU's and Motherboards/Chipsets. Many experts suggest most systems don't need more than 400 watts as long as the PSU is a quality make.

    This would be a great addition with more modern hardware especially now that we have SSD's and M.2 hard drives.

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