Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
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.
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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.