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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  • 7Enigma - Monday, September 22, 2008 - link

    20% greater system power? or just for the cpu? If just the CPU what did it equate to system-wise if you don't mind me asking?

    In an earlier post I mentioned a high-end PSU possibly being better than the mid-grade if you were going to moderately overclock (it was right at the overlap point under heavy load) from a sound and efficiency standpoint. But that was assuming an increase of 20-30% overall.
  • CK804 - Monday, September 22, 2008 - link

    Finally, an article from Anandtech that will really open people's eyes on how much power they actually need. Reply
  • Beenthere - Monday, September 22, 2008 - link

    When purchasing a PSU, ignoring the importance of PSU quality and output, in favor of noise and efficiency is foolhardy.

    Many PSUs do not provide clean power or the rated power - especially under heavy laod. In additional while the article touched on it, depending on the 12V rail design, many PSUs can't deliver the proper power (wattage) to the 12V rail(s) even though the PSU total wattage rating may be more than sufficient . While I'm all for green it is always better to buy a quality PSU that delivers at least 20% more power than you current needs, to provide update headroom and maintain good PSU efficiency and low noise.

    If you're not comparing PSU quality, power output per rail and warranty before considering efficiency, noise and cost, then you've missed the point of buying a proper PSU. While most folks do not need a 1000W PSU, many need a quality PSU that can deliver the correct power to each rail and a PSU that will last.
  • 7Enigma - Monday, September 22, 2008 - link

    Wouldn't a high efficiency PSU by design be a quality PSU? Generally inferior parts/design are the reason for poor efficiency... Reply
  • mindless1 - Thursday, September 25, 2008 - link

    High efficiency doesn't automatically mean high quality per it's own ratings, and an old design not attempting to have high efficiency can still be using reasonably good quality parts and design, unless all your criteria revolve around efficiency being a necessary factor before you'd call a PSU "quality".

    Take server PSU for example, many don't have such high efficiency but many are higher quality than those used in PCs.

    Do you realize that more elaborate filters will reduce efficiency? To some extent, trying to maximize efficiency limits how much quality can be present.
  • marc1000 - Monday, September 22, 2008 - link

    and it is powered by a old Pentium-D 945 (3.4ghz, 90w TDP, and it gets quite hot actually). I also have 1 HDD, 1 DVD-RW, 2GB of ddr2 ram, and one Radeon 3850 512mb.

    I was in fact using a high-quality 250W PSU, the one that came bundled with the system (I believe it is high-quality because of the build quality and the clear specs, and also because it is a system I bought from HP with "free upgrades option", so I upgraded the CPU+GPU by my own. I wanted a Core2Duo, but my mobo will not accept it.)

    whatever. the 250w PSU was working fine, even when I ran old games or 3dmark01. but on 3dmark03/05/06 and newer games, the system was turning itself off after 5 or 10 minutes. so i bought this very quiet AKASA 300w PSU with a single PEG, and now I have a relatively quiet computer that works just fine with no power problems.

    this is quite a good gaming machine, if you want to know. I am OK with 20/30 fps, as I am not a hardcore gamer. and I can play GRID at 1680x1050 with almost everything high and 2xAA at this frame rate. also Crysis run fine with everything to medium at this resolution and frame-rate (but this is not so enjoyable because it is a fast-paced FPS). I know that my CPU is the bottleneck, but maybe next year I will change the mobo+CPU.

    It is a modest PC, with a modest PSU, for a modest gamer! =)
  • Insomniac - Monday, September 22, 2008 - link

    You said the Corsair VX450W performs best of the low power usage system PSUs. But looking at the charts, it seems the Amacrox Calmer 300W is the best. Its efficiency curve is the highest through the range and its noise curve is the lowest through the range. Was this an oversight or was there a reason this PSU would not work for the midrange system?

    If it was, that seems like a great PSU. It would be close to the other mid-range PSUs in efficiency and the best for noise. It would be great for a low power system, but has quite a bit of headroom as well.
  • Christoph Katzer - Monday, September 22, 2008 - link

    Since it is passively cooled it has a very limited usability... If you run a few fans it won't be a problem but then you still have the huge price difference between both units. Reply
  • Insomniac - Monday, September 22, 2008 - link

    Thanks for the information! Reply
  • duploxxx - Monday, September 22, 2008 - link

    Very nice article it really explains the desires and needs what to buy for PSU altough i am missing some top psu's like seasonic for example.

    Only unfortunate is that some measurements of hardware are way out of range... especially in the motherboard parts.

    And if you want to be stay out of who is best... you know the always existing rival that a site has a preferred vendor, take the latest hardware from both sides, if not leave it out.

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