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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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.