Efficiency Explained

All power supplies have a specific efficiency curve, which we show in our reviews. As an example, we will use the Cooler Master UCP 900W power supply. Here is the efficiency curve we measured during testing:

Efficiency is the output power divided by the input power, as there is a certain amount of power lost during the AC to DC conversion. The x-axis shows the power supply load in Watts and the y-axis shows efficiency. Let's include our three sample systems in the chart see what sort of efficiency they would get with this power supply.

The first system causes this high-performance power supply to only run at 73% to 81% efficiency, depending on input voltage. Obviously, there's absolutely no need for a 900W power supply if you're running this type of computer.

The midrange system looks quite a bit better, allowing the PSU to run at 80% to 88% efficiency, although the latter only occurs at maximum load. Considering the vast majority of systems rarely run at 100% load most of the time, real-world efficiency will average closer to 82%. Office work and Internet surfing in particular will be at that level.

For the third system, a 900W power supply actually might start to make sense. It's still more than you need, but having a bit of extra room to grow isn't a bad idea. This system idles at over 300W, so it achieves a minimum 86% efficiency with 120VAC. When running a game or other demanding task, the PSU is finally able to reach its potential and provide 89% efficiency with 230VAC (or 87.5% with 120VAC).

The quick summary then is that if you don't have a system that uses 350W of power when idle, it's probably not worthwhile to purchase this type of power supply. Our high-end sample system more or less meets this qualification, and if you were to take such a system and overclock it, these high-end power supplies are actually required. The 8800 Ultra is one of the most demanding graphics cards currently available; however, the GTX 280 appears to require even more power, making that another candidate for this sort of PSU. (Unfortunately, our power supply testing labs didn't have the latest GPUs available for testing.)

Building Three Sample Systems What about Noise?
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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?
  • internetrush - Saturday, June 6, 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.
  • Christoph Katzer - Saturday, August 8, 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?
  • JohnMD1022 - Sunday, March 15, 2009 - link

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


  • 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.
  • Christoph Katzer - Thursday, October 30, 2008 - link

    I will have a review up soon!
  • 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
  • 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.
  • 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...

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