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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BernardP - Monday, September 22, 2008 - link
The following paragraph from the article has me puzzled:"It's important to have one 12V rail supply the CPU with power and the second rail for the PCI-E slots and 6-pin connector. Unfortunately, many companies make a tremendous mistake when it comes to power distribution. We have seen several power supplies that use one 12V rail for the 6-pin PEG connector and then a second 12V rail for the CPU and 24-pin ATX connector. That means if you have a graphics card that doesn't include a 6-pin jack, both the CPU and GPU will use the same 12V rail for power. In this case, the second 12V rail goes completely unused, and users risk drawing too much current on the remaining 12V rail."
I have an Antec Eartwatts 380. How can I find out if Antec has made the tremendous mistake or not? I want to make sure that the 6-pin connector and PCI-E slot are on their own 12V rail. Antec litterature on this PS says:
"Dual 12V outputs: 12V2 for motherboard and peripherals, 12V1 for processor"
It would seem OK, assuming "peripherals" includes the 6-pin connector.
7Enigma - Monday, September 22, 2008 - link
What is your graphics card? From the article it seems to me they are saying if you do NOT have a card that requires a 6-pin PEG connector then you could possibly have an issue. If you have a card requiring a PEG connector you don't have to worry.BernardP - Monday, September 22, 2008 - link
Exactly. I am planning to add a 9500GT to my existing system (integrated graphics). No power connector on that card.Dribble - Monday, September 22, 2008 - link
http://extreme.outervision.com/powercalc.jsp">http://extreme.outervision.com/powercalc.jspSeems to return sensible values, and not only does it cover pretty well every component you might come across, but it also understands overclocking, over volting, and allows you to enter a value for capacitor ageing.
Also, here is a thread which someone has helpfully listed real power requirements (as given in reviews) for pretty well all graphic cards on the market right now:
http://archive.atomicmpc.com.au/forums.asp?s=2&...">http://archive.atomicmpc.com.au/forums.asp?s=2&...
drank12quartsstrohsbeer - Monday, September 22, 2008 - link
Hey Guys: Remember that decibels is a logrithmic scale of measurement! Using a linear scale on the graph leads to inappropriate conclusions being drawn from the data.7Enigma - Monday, September 22, 2008 - link
I don't think it does. Maybe a quick *note* at the beginning of the acoustics section mentioning its logrithmic, but it is very easy to read a linear scale.Also, the majority of the tested systems fall well below the floor of most systems (20 decibels), so it is a moot point anyway.
gmkmay - Monday, September 22, 2008 - link
I'll start off by saying good article, however I would have liked to see a few additions.Other than the aforementioned new cards and overclocking information I think it would have been helpful to include common watercooling pumps and case/system fans. There is most likely a large enough set reading this that would have liked to see those added.
The problem with the power supply issue is you have to be really careful not to get something too weak...and its really easy to forget a few small items that can quickly add up (for instance 2 pumps, 8 120mm fans, etc).
mindless1 - Thursday, September 25, 2008 - link
Nobody building a PC needs 8 x 120mm fans. Let's suppose you throttle down the fans enough that you might actually have good use for so many to have them all at very low RPM. That would tend to cause under 150mA per fan or barely over 1A total, a relatively trivial amount of power considering that even spinning up any one hard drive causes a larger momentary spike.A couple pumps shouldn't use all that much power either, but if you're pouring enough money into the system to have it that elaborate then why would you be on the fence about choosing a marginally capable PSU versus one with plenty of reserve power to the point where 3A one way or the other isn't a factor?
Anubis - Monday, September 22, 2008 - link
would be interesting to see numbers on just how much power OCing pulls over a non OCed systemChristoph Katzer - Monday, September 22, 2008 - link
I had a QX9770 just slightly overclocked with basic bios functions (for an oc-noob like me) and already then it had an increased power draw of 20% at full load compared to normal.