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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kuraegomon - Monday, September 22, 2008 - link
Aargh. The most important argument for ensuring that your PS has plenty of headroom is ... lifespan!The most knowledgeable PS people out there will all tell you the same thing: running even a quality PS at consistently more than 80% or so of its rated output is all but guaranteed to reduce its operational lifespan. It's also a catch-22 because the longer a PS is run at high load, the less the maximum load it can support becomes. This isn't a terribly quick process, but quite a sure one. Track down any number of JonnyGuru's comments/reviews out there for more info.
Do all the math stated in this article, and figure out what your idle and load draws approximate too, then make sure you've got 30% headroom on top of your load requirement. This is BEFORE taking into account any expansion plans. Also, try to remember that 850 PCP&C supply described here STILL isn't being used the way a power user uses their system. It's spending more time at or near peak load, but it's also quite likely spending a fair bit of time on the shelf.
7Enigma - Monday, September 22, 2008 - link
What is your point? In the article the PSU's for each category are well under 80% utilization. If you look back at the charts, in the rated range for each system:-low end is utilized <50% of rated wattage
-mid-range is utilized <75% of rated wattage
-high-end is utilized <80% except for the Neopower Blue (which honestly looks pretty crappy both from a efficiency and sound standpoint)
That is across the board. In each category the higher-rated parts are obviously utilized less than those percentages.
Even the comments below each chart bares this out. For the low end, for example, they state that a 250w PSU would be perfect, while even a 200w would suffice. With a total system draw of 140w, the 250w would be near 50% utilization (56% if you want to be picky), and the 200w would still be <75% utilization.
I don't see fault in this article from that standpoint.
vlado08 - Monday, September 22, 2008 - link
The problem is that even if you calculate the expected power draw of your system you have to trust the label on the power supply and to be sure that if it says 500w then it is so. Well then you just end up to trust the trade mark or some reviews for the model you are going to by.Or you trust somebody who is going to assemble your new computer for you.
7Enigma - Monday, September 22, 2008 - link
Thank you very much for this article. As someone building a system by the new year I appreciate it greatly!One interesting thing is that there are times where the higher wattage supplies actually make more sense due to efficiency (and probably more connectors/warranty/etc.).
The Enermax Pro82+ 625w is definitely the best mid-range you have listed IMO for a stock system (ie non-OC'd), but for someone looking to OC their system I think the Zalman850 from the high-end section is probably the better buy (both efficiency and soundwise). There is a crossover point very close to idle power levels (if you take into account another 25-50w for an OC'd system), and so anything above idle will have better efficiency and at load quieter levels.
But I haven't checked the price difference, which I'm assuming is quite large. A 1-2% efficiency difference between the Enermax and Zalman is probably not worth the increase in price from both a ROI (from power savings and increased case temp from inefficiency).
Thanks again for the great review!
vlado08 - Monday, September 22, 2008 - link
In the article you didn't mention how did you measure the power draw of different components for example the CPU or the draw from PCI express? And haw did you test the efficiency of the different power supplies?Don Tonino - Monday, September 22, 2008 - link
I found the remarks concerning the efficiency charts a bit misleading... why give the range of efficiency of the high end system, for example, as between 85% and 89%, when the first number refers to the efficiency with 90VAC? the numbers given out are not consistent, as the systems at 230VAC show in reality the following efficiencies approx:low end - 74 to 80 %
midrange - 82 to 88 %
high end - 87.5 to 89.5%
Based on the numbers above, the PSU is actually quite well suited to the high system as the efficiency changes by a meager 2% between idle and load. It would be even better with some extra load, so to place the idle/load range between 450 and 700 W.
As far as the point to make is to show how efficiency changes with the load, it would have been as meaningful to give data just for the 230VAC, as it was already stated that efficiency with 120VAC or 90VAC would be even lower.
Insomniac - Monday, September 22, 2008 - link
The range isn't for 90VAC to 240VAC, it's to cover the idle load to full load range of the sample system.Don Tonino - Monday, September 22, 2008 - link
Check the chart. For every system the lower efficiency, the one given for the sistem at idle, has a value that at that particular power load (respectively given as 90W, 168W and 310W) lies on the red line, the one representing efficiency of the PSU when running at 90VAC.This is most evident if you take the high end system, which is stated will make the PSU run at an efficiency between 85% and 89%; those values, if you move on the blu line (PSU running at 230VAC) means a power load between 200W and 650W).
Giving the idle efficiency with the PSU running at 90VAC and the load efficiency with the PSU running at 230VAC gives a much higher change in efficiency than real. The only real meaning for it would be to say: "with such a system and such a PSU you will have an efficiency between A% and B%, based on the current the PSU is running on"... and I seriously doubt that anyone at home have an electrical system that changes VAC on the run.
Insomniac - Monday, September 22, 2008 - link
I see what you are saying now. I misunderstood what you said before. It seems a table would be better to show the efficiency range for each, or the values for one curve only (the article seems to say it was supposed to only be 230 VAC).JarredWalton - Monday, September 22, 2008 - link
Sorry for the error - not sure how we missed that, but yes the efficiency with the high-end system and UCP 900W is higher than stated initially. Must have been confused with the other systems, but I'll correct the text now.