Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Introduction
Anyone building a computer system should eventually pose the question: How much power does the system actually require? This is an important consideration, since it's impossible to choose an appropriate power supply without actually knowing the demands of your system. Unfortunately, many users take the easy way out: just grab a 500W power supply and call it good. If you really want to be safe, you can even grab on 800W PSU... or if you plan to run multiple graphics cards perhaps you really need a 1000W unit, right?
If people really took the time to examine system power requirements, we would see a tremendous increase in sales of 300W to 400W PSUs. The truth is that the vast majority of systems would run optimally with such a "small" power supply. Even if you're running SLI/CrossFire, you don't actually need a 750W power supply. (Of course, we recommend purchasing a good quality power supply, as there are certainly "750W" PSUs out there that can't reliably deliver anywhere near that much power.) To help dispel some myths relating to power requirements, we've put together a couple of charts.
| GPU Power Consumption* | ||
| Manufacturer | Idle | Load |
| NVIDIA GeForce 9600 GT | 49W | 107W |
| NVIDIA GeForce 8800 GT | 64W | 115W |
| NVIDIA GeForce 9800 GTX | 79W | 116W |
| NVIDIA GeForce 9800 GX2 | 90W | 179W |
| NVIDIA GeForce 8800 Ultra | 100W | 186W |
| ATI Radeon HD 3650 | 17W | 32W |
| ATI Radeon HD 3850 | 53W | 82W |
| ATI Radeon HD 3870 | 62W | 92W |
| ATI Radeon HD 2900 XT | 67W | 104W |
| ATI Radeon HD 3870X2 | 55W | 130W |
* Actual power consumption for the graphics cards only. Results taken at idle on the Windows desktop and under full load running the Fur benchmark.
| CPU Power Consumption** | |||
| Manufacturer | Idle (EIST or CnQ Enabled) | Idle | Load |
| Intel Core 2 Duo E4500 | 14W | 17W | 36W |
| Intel Core 2 Duo E8500 | 18W | 22W | 43W |
| Intel Core 2 Quad Q9550 | 19W | 23W | 60W |
| Intel Core 2 Extreme QX6850 | 29W | 32W | 103W |
| Intel Core 2 Extreme QX9770 | 26W | 56W | 86W |
| AMD Athlon 64 X2 5000+ | 33W | 47W | 89W |
| AMD Athlon 64 X2 6000+ | 25W | 74W | 160W |
| AMD Phenom X3 8750 | 50W | 67W | 86W |
| AMD Phenom X4 9600 BE | 29W | 36W | 101W |
| AMD Phenom X4 9850 BE | 38W | 53W | 126W |
** Actual power consumption for just the processor. Results taken at idle on the Windows desktop with either EIST/C&Q enabled or disabled, and full load generated using BOINC.
| Chipset/Motherboard Power Consumption*** | |
| Platform and Chipset | Load |
| Intel P35 (775) | 37W |
| Intel P965 (775) | 39W |
| Intel X38 (775) | 52W |
| Intel X48 (775) | 40W |
| NVIDIA 680i (775) | 46W |
| NVIDIA 790i (775) | 51W |
| NVIDIA 750i (775) | 59W |
| NVIDIA 780i (775) | 69W |
| NVIDIA 8200 (775) | 29W |
| AMD 690G (AM2) | 34W |
| AMD X3200 (AM2) | 35W |
| AMD 770 (AM2) | 40W |
| NVIDIA 570 (AM2) | 40W |
| AMD 790FX (AM2) | 42W |
| AMD 790X (AM2) | 43W |
*** Actual power consumption for the motherboard and chipset. Idle and load power do not differ by any significant amount.
Top-end graphics cards are clearly one of the most demanding components when it comes to power requirements in today's systems. Only heavily overclocked CPUs even come close to the same wattages. Note that the above chart only includes last generation cards; NVIDIA's latest GTX 280 requires even more power.
Looking at the processor side of the equation, Intel's Core 2 Duo/Quad/Extreme CPUs in general have very low power requirements. AMD's latest Phenom processors aren't far behind, however, especially in light of the fact that they include the memory controller rather than delegating the task to the chipset. We should also mention that part of the reason for the extreme power requirements on the X2 6000+ come from the use of an older 90nm process.
Naturally, motherboards also require a fair amount of power. Current motherboards average around 47W for socket 775 and 39W for socket AM2/AM2+, but features and other factors can heavily influence that number. Outside of their IGP solution, NVIDIA's chipsets tend to use more power than the competition; AMD chipsets on the other hand typically require less power. Again, numerous other aspects of any particular motherboard will impact the actual power requirements, including BIOS tuning options.
Hard drives and optical drives account for another 10 to 20W each. However, remember that hard drives are a relatively constant 10 to 15W of power draw (average is around 12W) since the platters are always spinning (i.e. idle), and movement of the drive heads during read/write operations (i.e. load) only increases power draw slightly. Optical drives on the other hand stop spinning when idle, requiring only about 5W, while during read or write operations they need around 18W.
RAM power requirements measured a constant 2W per DIMM, regardless of capacity (though clearly not including FB-DIMMs). That figure is estimated, unfortunately, as we could not measure DIMM power requirements directly; we measured power draw with two DIMMs and then again with four DIMMs to arrive at the reported figures. It's also not possible to easily separate memory power requirements from the motherboard and chipset, as they share many of the same power connections from the PSU.
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Christoph Katzer - Monday, September 22, 2008 - link
Yes I guess that was the problem. Thanks for pointing that out!ViRGE - Monday, September 22, 2008 - link
Christoph, thanks for the excellent article. There is one thing I noticed however that has me a bit confused. In a few of your video card tests, you have the cards pulling upwards of 100 watts from the PCIe slot. It has been my understanding, and is mentioned in AT's X38 chipset article*, that the limit for a PCIe slot is 75W. Your results put this value in contention, so I'm not sure what to believe at this point.Is it that I am mistaken that the limit is 75W, or are your video cards pulling too much power from the slot, or is there a problem with the data?
Christoph Katzer - Monday, September 22, 2008 - link
Thanks. The data is correct since the cards draw power through the PCIE slot and the additional PEG connectors (6-pin or 8-pin). Towards the end of the article you will find a list which card drew how much power from the PCIE slot and the PEG connector. 75 watts is however correct for the older PCIE slots, PCIE2.0 can draw up to 150 watts.mobutu - Monday, September 22, 2008 - link
Where is AMD 4850?Also almost all the graph cards used are kind of old, previous generation.
Dobs - Monday, September 22, 2008 - link
Here is a link to a comprehensive list of all graphics cards.Dobs - Monday, September 22, 2008 - link
damn... link didn't workCopy and paste :)
http://archive.atomicmpc.com.au/forums.asp?s=2&...">http://archive.atomicmpc.com.au/forums.asp?s=2&...
Clauzii - Monday, September 22, 2008 - link
Nice lists, thanks :)Christoph Katzer - Monday, September 22, 2008 - link
As stated I didn't have all of the new stuff here and this article anyway focuses primarily on power supplies and how to choose them and not the power consumption of the components... but it might come soon, who knows.LTG - Monday, September 22, 2008 - link
>>our labs don't have the latest GPUs available for testingIsn't this kind of a bad thing for a site with so many enthusiast readers?
First two caveats:
1) There are a lot of article bashers out there who nit-pick, give rude feedback, or are just plain wrong, I don't support these guys or want to be one.
2) The article written was very good and helpful, thank you.
However it doesn't matter that the article "primarily focused on how to choose power supplies".
- Leaving out a GTX 280 was a big omission. Even better to have an stock OC'd product.
- Leaving out very basic overclock numbers was a big omission. For example it's very common for readers to have an Intel QC clocked at 3.6Ghz since it's so doable.
Having the last two data points would be very helpful for those building top systems, and would even be interesting for those who are not.
7Enigma - Monday, September 22, 2008 - link
As mentioned, the article is about the myth that we all need 1k PSU's for the "best experience". Bottom line this article shows that the vast majority of people do not need more than a 500-600w psu. How is leaving out any stock product an omission? Just look at the TDP of the product and go from there. All stock products will fall within a narrow range.Once you get to OC'ing I also think this is a difficult thing to do. Individual chips, even from the same batch can have very different properties and require different voltages to reach the same speeds. I'm sure if the author had put that his Intel quad drew 120w at 3.6GHz, someone would have complained that their's took 135w, and someone else only 110w...Once you go above stock, it is not cut and dry and as the article states you should have a 20-30% extra buffer if you wish to OC.
I feel the scope of the article just doesn't require these extra requests. Sure it would be nice to see just how crazy of a system you can make (if I remember correctly they did that a while ago to actually use the >1000w supplies), but this article was to show what a normal base, mid-grade, and high-end setup would require.
I've complained in the past about articles, but this one I don't see any serious faults.