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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  • JarredWalton - Monday, September 22, 2008 - link

    If you look at all the included components, it should be pretty clear that the latest parts won't be drastically different from the tested components. Sure, 4870 might use 30W more (or 30W less) than the 3870, and the P45 might use +/-10W relative to the P35. Does that really change anything with the information the article conveys? I don't think so. Midrange PCs are still going to use 150-250W for the most part, whether with last year's components or with the latest stuff. If you want to look at the top-end, then GTX 280 will use more power than the listed GPUs, but even then you're not going to break 600W without overclocking. Reply
  • Martimus - Tuesday, September 23, 2008 - link

    Thanks. I have since calculated out what is needed for the processor and chipset (for a E8400 and P45 MB) and it came to about 8A. I went on the AMD website to find what the current draw was from the 4870. It doesn't say, but does recommend a 500W PSU. I am a little concerned though, because the EA500 isn't on their approved PSU list from Antec, and I have had issues with their power supplies before, plus they only have 17A available on each rail. that should be fine, but I would like to make sure I am not loading the 12V rail too heavily as well. If those components don't work (those are my plan for the moment), then I can always adjust, but I would like to have that peace of mind before I make the purchases. (the computer is a present, and I bought the Sonata III to save money because I heard good things about the Earthwatts brand, but now I am starting to get worried about this particular model.) Reply
  • Matt Campbell - Monday, September 22, 2008 - link

    I would also point out that in addition to all of the data Christoph has pulled together, we have some power consumption numbers on overclocked systems with GTX 280s as well, which seems to be one area people are asking for.

    4.0 GHz QX9770 with SLI GTX 280s: 579W Max
    4.0 GHz QX9650 with Triple 8800 Ultras: 671W Max
    Reply
  • JarredWalton - Monday, September 22, 2008 - link

    Those figures are power at the outlet, though, right? And they're not tested in the same fashion as what Christoph did. Still, if you max out at 671W at the outlet, even with 88% efficiency you're only using 590W - nowhere near 1200W, which is what NVIDIA certifies for 3-way SLI. Quality over quantity, naturally, but there really aren't many terrible 1000W PSUs out there (which is why they all cost over $200). Reply
  • Torched - Monday, September 22, 2008 - link

    For all the hullabaloo about the 12v Rail why nor just recommend a PS that has a single rail. More and more manufacturers are going in that direction anyways since it eliminates the whole power trapping issue. Reply
  • gramboh - Monday, September 22, 2008 - link

    Good article. The underlying point is that you do not need 1000-1500W PSUs to run your system like I see many people claiming you do for SLI setups. I see lots of people with 700-800W PSUs with one graphics card. Insane.

    I've been running a Corsair HX520 (520W) for a year and a half now. My current config is:

    Asus P5B-Deluxe (P965) mobo
    Q6700 G0 @ 3.3GHz (1.42v actual voltage reported by Speedfan)
    4x1GB PC2-6400 DDR2 memory
    EVGA GTX280 video card overclocked (675-1350-2422)
    3 hard drives (7200.11 1TB + 2x 7200.10 500GB)
    1 optical (18x Samsung DVDRW SATA)
    3 case fans ~1500rpm + CPU fan 1200rpm
    SB X-Fi XtremeGamer audio

    Zero issues and I'm sure I have headroom to spare. Some people on forums told me I should upgrade my PSU to at least a 650W Corsair or 750-800 of other brands to run the 6700/GTX280, just goes to show people have fallen for the marketing hype of PSUs.
    Reply
  • cesthree - Monday, September 22, 2008 - link

    I have had a few different systems in the last 6 months. Using the Zalman MFC-2 I can see the wattage being used in real-time. How accurate it is, I do not know, but it gets me in the ballpark.

    All systems have had the same 4 WD1600YS HDD, Lite-On DVD burner, 700W OCZ GameXStream PSU, and OCZ DDR3 1333Mhz 2x1 GB RAM. I also run 4 x 120mm fans including the one mounted to my TRUE-120.

    Set-ups:

    1. EVGA 790i Ultra (JUNK) w/ Q6600 @ 3.2Ghz 1.35VCORE, 1400Mhz FSB, 8800GTS 320MB SLI
    Idle Load: 225W, Prime Load: 275W, 3DMARK06: 325W.
    Never saw higher than 400-425W during highest loads.

    2. DFI X48-T3RS (PWNING 790i ULTRA) w/ Q6600 @ 3.0Ghz 1.32VCORE, 1333Mhz FSB, EVGA 9800GTX
    Idle Load: 200W, Prime Load: 250W, 3DMARK06: 290W.
    Never saw higher than 350-375W during highest loads.

    3. DFI X48-T3RS w/ E8400 @ 3.0Ghz 1.245VCORE, 1333MHZ FSB, EVGA 9800GTX
    Idle Load: 175W, Prime Load: 200W, 3DMARK06: 250W.
    Never saw higher than 300-325W during highest loads.

    BTW, all Watts are averages. Prime Load is averages between blend testing and small FFT's.

    I really like the meter on the MFC-2. If it is at all accurate, then it supports the logic that it isn't always necessary to have a 2000KW PSU.

    I could see needing a higher wattage, QUALITY PSU, for CF or SLI + the latest GPU'S, non-extreme CPU, with everything OC'd 25-50%, and maybe a single loop WC as well.

    Just my 2 cents.
    Reply
  • Spacecomber - Monday, September 22, 2008 - link

    I was wondering if it might have been helpful to add some comments on the importance (or not) or having 8-pin 12-volt motherboard connectors, instead of the more standard 4-pin motherboard connector. I don't think that this motherboard connector, which I think largely powers the CPU, was given much attention relative to the discussion of the additional 4-pin connector for the main motherboard connector (20 + 4) and the PCI-E power connectors.

    Overall, this was a very good article. I like how it places technical details about power supplies into a context of everyday use.
    Reply
  • CSMR - Monday, September 22, 2008 - link

    The power consumption data on page 1 is completely overblown.
    The tables claim to represent actual power consumption of processors and chipsets but the figures are completely exaggerated.

    There are even people who run whole systems on one or two of the chipsets listed on less power than the power the article claims for just the chipset.

    Here are actual CPU measurements from behardware and xbitlabs:
    http://www.xbitlabs.com/articles/cpu/display/intel...">http://www.xbitlabs.com/articles/cpu/display/intel...
    http://www.xbitlabs.com/articles/cpu/display/intel...">http://www.xbitlabs.com/articles/cpu/display/intel...

    I haven't seen measurements of chipset power but here is a list of TDPs:
    http://www.silentpcreview.com/forums/viewtopic.php...">http://www.silentpcreview.com/forums/viewtopic.php...

    Must fix this soon as Anand needs to keep its reputation for good information.
    Reply
  • Zoatebix - Monday, September 22, 2008 - link

    Seriously. This guy didn't go too far our of his way to make a 30-40 watt system: http://www.silentpcreview.com/forums/viewtopic.php...">http://www.silentpcreview.com/forums/vi...tdays=0&... Reply

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