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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  • LTG - Monday, September 22, 2008 - link

    You can't just look at the TDP's, that's the problem.

    When you overclock the power demands can increase in a very non-linear way.

    So for example I can't plan for one of the most popular 280 cards like the EVGA FTW 280 GTX. It's 11% core over clocked, what does that mean at the outlet? No good way to guess.

    Same for a 3.6Ghz QC CPU - this is a very common overclock, yet there is no direct way to know it's power requirements.

    I'm just saying this is not esoteric information, this would be data people really could use and can't get from the manufacturer.

    Reply
  • xaris106 - Friday, November 07, 2008 - link

    But you can. All you need is stock power consumption at load(Pstock), stock voltage(Vstock) and stock frequency(Fstock) The oc power is then:
    Poc = Pstock * (Foc/Fstock) * (Voc/Vstock)^2
    Reply
  • nubie - Monday, September 22, 2008 - link

    Unless they edited this, you are operating on a false assumption.

    (Unfortunately, our power supply testing labs didn't have the latest GPUs available for testing.)

    A power supply testing lab doesn't need to have every component on hand because it uses a test bench to load the supplies.

    Great article, way to dispel myths, I guess since I only plan to overclock with a single video card and one or two hard drives my PCPower Silencer 470 will be enough power for many years to come (which is what I hoped when I bought it, the only downside is the single 6-pin for the video card, when it can clearly handle much more.)
    Reply
  • JarredWalton - Monday, September 22, 2008 - link

    Just for the curious, AnandTech staff is scattered far and wide around the globe (well, at least the US and Europe). I'm west coast, Wes is east coast, Anand and Derek are in NC, Gary is in TX, and we have Johan and Liz in Belgium with Christoph in France. (That's not everyone, but you get the point.) Since we tend to focus on our own areas of testing, Derek and Anand have the most CPU/GPU hardware, I have laptops and displays, Gary has motherboards, etc. I can definitely say that Christoph isn't the only one without 48x0 and GTX 2x0 hardware. [Pardon me while I go cry in a corner now....] Reply
  • hyvonen - Wednesday, June 03, 2009 - link

    Oh, so in order to get this power draw info on more components, I should beg Anand? :) Reply
  • LTG - Monday, September 22, 2008 - link

    Totally understood, many companies now days are distributed and can't have every physical resource available to every person.

    However I would volunteer to send Christof a new 280 GTX to test if he decides it's worth it.

    Serious - Just please send it back whenever finished :). And I waive all claims if it is accidentally fried by that fancy Chroma thingy.

    Reply
  • ineedaname - Tuesday, November 02, 2010 - link

    This article is well written and tried to put real life numbers and situations to the test on PSUs.

    However i felt that they should mention one other thing for people who are novice to computers. They should mention that just because a PSU is rated for 500w it doesn't necessarily mean that it will do the job even if your computer will only suck about 150w max. Because a 500w psu that comes along with a $50 case just won't do the job. Not because of the wattage rating but because they use crappy parts and workmanship; it'll just die in 3 months when the warranty is over.
    Reply

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