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.

Building Three Sample Systems
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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.
    Reply
  • 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. Reply
  • 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. Reply
  • Dribble - Monday, September 22, 2008 - link

    http://extreme.outervision.com/powercalc.jsp">http://extreme.outervision.com/powercalc.jsp

    Seems 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&...
    Reply
  • 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. Reply
  • 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.
    Reply
  • 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).
    Reply
  • 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?
    Reply
  • Anubis - Monday, September 22, 2008 - link

    would be interesting to see numbers on just how much power OCing pulls over a non OCed system Reply
  • Christoph 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. Reply

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