Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Final Thoughts
Building a system these days feels a lot more complex than it was a few years ago. There are many more components in the market, with new ones announced every week, and there's also a lot of overlap. It's nice that some manufacturers are now including advice on power requirements on their products, but that doesn't mean you should take this information at face value. A processor vendor might list a TDP of 150W, but that doesn't necessarily mean that particular processor will actually use 150W. It is more of a rough indication, but at least it's better than nothing.
We have tested processors from AMD and Intel and we have to conclude that the Intel parts are usually far below the listed TDP, where is the AMD parts are sometimes higher. The same applies to graphics cards, but in this case we have a huge difference between the various loading mechanisms. For example, we couldn't reach maximum load on an 8800 Ultra using any current DirectX 10 game, but the Fur benchmark was able to do so. To achieve a similar load, you need to balance frame rates, anti-aliasing, anisotropic filtering, and resolution. High frame rates without AA/AF typically don't put as much of a load on the GPU(s), and likewise high AA/AF settings with low frame rates usually result in lower power consumption. In testing, it seems like maintaining 80 to 100 FPS while increasing AA/AF levels resulted in worst-case power consumption -- though obviously that can vary by application/game.
Something else you need to keep in mind when building a new system is your hard drive power requirements. Most power supplies provide information about their power consumption on the label, although they list current rather than wattage. The power requirements for a hard drive are quite low compared to processors and graphics cards, but remember that hard drives typically need three times as much power during startup as they use during normal use. This only occurs for a split second, so this spike is only visible using an oscilloscope. However, this can cause problems with certain power supplies in combination with larger disk arrays.
Hopefully this article has helped to shed some light on what factors need to be considered when choosing a power supply. Our tests have certainly showed that most components to use nearly as much power as you might think. Look at the tables we've listed on the first page, and then use that information to figure out what sort of power supply you actually need.
Our three sample systems show that few if any people actually need anything more than 850W power supply, especially if you can get an appropriate number of connectors. About the only reason you should consider anything more is if you want to keep your power supply at a lower load, since optimal efficiency and lower noise levels usually come at around half the rated output. Overclockers might also want a bit of extra headroom. Otherwise, there's really no need for a 1000W power supplies, and certainly not those with even higher ratings.
Our power supply reviews include efficiency and noise level charts at various wattage loads. Using these, you can easily compare your own system to see what sort of efficiency and noise you would get. You might be surprised to learn that your intended high-end setup really won't make much use of that spiffy new 1000W PSU; you might be greener than you think!
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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.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...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.Insomniac - Monday, September 22, 2008 - link
Thanks for the information!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.