Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Connectors and the 12V Rail Issue, Cont'd
As a second example, let's take our high-end system. This requires more power, making it a little more difficult to find an appropriate power supply -- especially if you want to run 3-way SLI. NVIDIA initially only certified 1200W power supplies for that sort of setup, but Corsair was eventually able to receive this certification for a 1000W unit. To run a 3-way SLI setup, we need at least six 6-pin PEG connectors, so let's start by looking at the number of connectors available on our high-end PSUs.
| PSU PEG Connectors | |||
| Manufacturer and Name | # of 6-pin PEG Connectors | # of 6/8-pin PEG Connectors | Total |
| Antec NeoPower Blue | 1 | 1 | 2 |
| Antec Signature | 2 | 2 | 4 |
| Silverstone Decathlon | 4 | 0 | 4 |
| Zalman ZM850HP | 2 | 2 | 4 |
| Enermax Pro82+ | 0 | 4 | 4 |
As you can see, none of the power supplies we listed are able to run more than two GPUs, since none of them have the available six PEG connectors. We still don't recommend 3-way SLI, has the advantages over two-way SLI are sketchy at best. Regular SLI also has the advantage of only requiring four 6-pin PEG connectors even for the highest-end GPUs. All of the listed power supplies can meet this requirement, except for the Antec NeoPower Blue.
For the second 12V rail issue, here's a look at the specific power requirements of our graphics card, the GeForce 8800 Ultra:
| GPU Power Requirements by Connector | |||||
| Vendor and Chip | Through 6-pin Jack |
Through PCI-E Slot |
Total Power | ||
| NVIDIA GeForce 8800 Ultra | 9.4A | 112.8W | 6.1A | 72.2W | 186W |
If you plan to run multiple cards, you can just multiply 186W by the number of cards in the system. We found during testing that the cards don't consume equal amounts of power. The first GPU always uses the same amount of power, whether in a single or SLI configuration. The second and third cards however require much less power, which also means that they are doing less work. In testing a variety of benchmarks and games, we saw that the second and third graphics cards only use 40% to 50% of the power of a single GPU.
One thing is obvious when looking at the power consumption of the single GeForce 8800 Ultra: the maximum power draw is only 15.5A, which you can get from a single 12V rail. Most higher-end power supplies have multiple 12V rails, which should make distributing power to your GPUs even easier. As long as the 24-pin ATX connector and the different 6-pin PEG connectors run on different 12V rails, none of the rails should end up with an excessive load. That takes care of the 300W power requirements mentioned by GPU vendors.
If you take the high-end system (with an optional third graphics card), you are looking at one of the most demanding systems available. Of course, you could always overclock the processor and graphics cards, which might increase power requirements by another 30%, but we'll leave that topic for another day. We were able to run this test system with a normal 850W power supply, and even with a reasonable load the PSU fan didn't make that much noise. Most of the high-end PSUs we've chosen for this article could also run a triple-SLI setup, provided they have sufficient connectors.
At the beginning of the year, we requested a special AnandTech Edition of PC Power & Cooling's Turbo Cool 860W power supply with six PEG connectors, specifically for running a triple-SLI setup. We have been using the power supply since then with three GeForce 8800 Ultra cards, an Intel Core 2 Extreme QX6850 CPU, and 12 Western Digital hard drives without any problems. Clearly, quality of construction and number of connectors are far more important than a silly wattage rating.
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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.