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How to Calculate Overclocked Power Consumption

Discussion in 'Die-hard Overclocking & Case Modifications' started by G L, Jun 30, 2004.

  1. G L

    G L Don Juan

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    The purpose of this thread is to show you how to calculate the wattage of your processor, and how to tell how hot that should make your processor run under load. This is just a compilation of other sources, so thanks to all the websites I've linked to.

    To begin with, note the formula near the middle of the following linked page. The author lists it as a "rule of thumb", by the way, so takes its results as an approximation only:

    http://www.amdmb.com/article-display.php?ArticleID=105&PageID=5

    OC Wattage = TDP * ( OC MHz / Stock MHz) * ( OC Vcore / Stock Vcore )^2

    The text "^2" means "squared".

    This formula is fairly self-evident. The stock/OC MHz is just how fast the processor is meant to run, and how fast you are in fact running it. Same with the vcore, which to any new people is the voltage that is run through the processor. Higher voltage means more stability, but also can lead to much higher power consumption, and if excessive, and early demise.

    The only term that needs explanation is the TDP, or Thermal Design Power. The TDP is meant to be the wattage of the processor at load. I say "wattage" because it is unclear if this is meant to corrospond most immediately to how much power is consumed in watts, or how much heat is produced in watts, but as near as I can tell the TDP is pretty much meant to indicate both.

    To get your processor's TDP, you must depend on AMD or Intel's own figures. In the case of AMD's XP line both maximum and typical are listed, and typical is what you want. Maximum is apparently if every single transistor were powered at the same time, which will never happen. Typical is meant to be the realistic maximum wattage.

    http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_739_7203,00.html
    http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_739_3748,00.html
    http://www.intel.com/design/Pentium4/documentation.htm?iid=ipp_dlc_procp4f+tech_doc&

    By picking the appropriate processor and scrolling through the pages, you should be able to find the TDP, and also the stock core voltage if you do not know it already.

    For this example I will use the Athlon XP 2500+ desktop processor. Its TDP and core voltage information are found on page 29 on the second link on the Athlon XP page.

    Athlon XP Model 10 512k cache 2500+
    1833 MHz
    1.65v
    53.7 watts TDP

    For this example I will use a relatively high overclock with a high core voltage, 2.5 GHz with 1.95v.

    OC Wattage = TDP * ( OC MHz / Stock MHz) * ( OC Vcore / Stock Vcore )^2

    OC Wattage = 53.7 * (2500/1833) * (1.95/1.65)^2

    OC Wattage = 53.7 * (1.36) * (1.18)^2

    OC Wattage = 53.7 * 1.36 * 1.39

    OC Wattage = 101

    So this 36% overclock and 18% core voltage increase has resulted in a 88% wattage increase.

    If the overclock were instead 2.4 GHz with 1.95v, this would result in 97.8 watts instead. If, on the other hand, we did the same 2.5 GHz but with only 1.85v, this would result in 92.8 watts. If it were actually possible to get that 2.5 GHz at the stock 1.65v, this would result in only 73 watts drawn. So the moral of this story is, don't raise your core voltage unless you absolutely need to, and a weak power supply may hold back your overclocking.

    Next: how wattage translates into temperature
     
    Last edited: Jul 1, 2004
  2. G L

    G L Don Juan

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    The measure a heatsink's ability to remove and dissipate heat is called its thermal resistence, which is expressed in C/W, or degrees per watt.

    This figure it often stated on the manufacturer's website, and many reviewers also do so as well.

    Here are some examples:

    http://www.dansdata.com/coolercomp_p9.htm
    http://www.dansdata.com/coolercomp.htm
    http://www.overclockers.com/articles373/
    http://www.frostytech.com

    An example is the Zalman 7000a-cu heatsink/fan combo.

    http://www.zalman.co.kr/eng/product/view.asp?idx=79&code=005009

    The manufacturer lists its C/W as .27 in silent mode and .20 in normal mode, the difference being the fan speed. We will stick with the latter figure. The .20 C/W figure means that the heatsink (and thus processor) will rise .2 degrees celsius for every watt of heat it draws off.

    Therefore, 100 watts of power would result in a temperature rise of 100 * .2 = 20 degrees. Zalman's .20 C/W figure seems a bit low, so I will rely on frosytech's instead:

    http://www.frostytech.com/articleview.cfm?articleid=1585&page=5

    They found a 24 degree rise above ambient at 100 watts, which translates into .24 C/W. Now we're able to calculate how hot our hypothetical 2500+ at 2.5 GHz 1.95v will be while being cooled by a Zalman 7000a-cu fan/heatsink.

    101 Watts * .24 C/W = 24 degrees above ambient

    The last step to determining how hot the processor will run is to add in the ambient. Let's say the air temperature in the case is 29 degrees Celsius. That means that the processor would end up at 24+29=53 degrees under load, which should be an acceptable temperature.

    The Zalman 7000a-cu was a convenient example because it comes with its own fan. With other heatsinks that do not, thermalright for example, the fan will come into play. Higher-powered fans will push the thermalresistence figure lower, lower-powered fans will push it higher.
     
  3. VashTheStampede

    VashTheStampede The Humanoid Typhoon

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    thanx again :)
     
  4. JohnMaclane

    JohnMaclane Ancient Guru

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    i was wondering if u can calculate the wattage needed for ur computer so ur computer wouldnt be underpowered
     

  5. G L

    G L Don Juan

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    This page will tell you the power cunsumption of an overclocked processor, otherwise refer to the Intel/AMD links to look up the figure. This tool will help with the rest of the system:

    http://www.jscustompcs.com/power_supply/

    Though I believe the DDR consumption may be a bit outdated. You may want to double that...
     
  6. JohnMaclane

    JohnMaclane Ancient Guru

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    thanks G L i hope it can help me with getting the right power supply needed for my new computer
     
  7. bigpawlu

    bigpawlu Guest

    hey joe if u need to calculate the power supply u cant just use cpu wattage there are other peripherals like thye mobo case fans which add to the overall wattage needed:D
     
  8. G L

    G L Don Juan

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    Yeah, the link I posted a couple messages ago is a tool that's meant to account for those variables.
     
  9. sak28wa

    sak28wa Guest

    Why worry about if your psu is enough for your sys? save a bit of $ and get one that'll be able to support your needs for the next few years and possible next several systems. I picked up a Enermax EG651P-VE (~550W) psu for about $150 USD.


    Can be found
    HERE!
     
    Last edited by a moderator: Jul 21, 2004
  10. Mrsteve4011

    Mrsteve4011 Master Guru

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    150 for a power supply............dont have that kinda money
     

  11. Jflo22

    Jflo22 Active Member

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    This might sounds stupid, but does anybody know if there is a formula for calculating how much Voltage is needed for certain OC'd CPU's?
    For Example: 3.2E P4 @ 3.6E, Stock voltage is 1.42, what would it need to be to cover the 3.6?
     
  12. G L

    G L Don Juan

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    I don't think there's any specific forumla, and even if there was something the problem is that different CPUs overclock differently, even if they're technically the same model. Processors have different OC capabilities just coming off the assembly line, but then newer batches will do better as well, sometimes substantially better. There are people here who have recent 3.2c P4s at 3.85 or 3.9 GHz on air, which used to require exotic cooling closer to when they first hit the market. To know what core voltage you'll need, the thing to do is just start overclocking slowly, and bump up the voltage when an OC fails and see if it helps you continue.
     
  13. adrenacrome

    adrenacrome Member

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    You can get these 500W beauty's for about $69.99 on Xoxide.com's X-hour...
    [​IMG]
    [​IMG]
     
    Last edited: Nov 29, 2004
  14. Atomic

    Atomic Guest

    Sanda can tell you the power rating in watts under the cpu and bios information module.

    wow...just ran a few numbers using that equation and appearently i was putting 159w through my CPU when the TPD is 89w when I made my highest score in sandra at 2.5ghz....sheesh....thats alot of juice.
     
    Last edited by a moderator: Jan 19, 2005
  15. owww... my head.... math ..... AHHHH!!!!!
     

  16. G L

    G L Don Juan

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    Unfortunately, this isn't as relevant as it used to be because both AMD and Intel are no longer nice enough to post specific power consumption figures for each processor. Now they just list a maximum number for the whole line, or for a group of processor speeds.
     
  17. BuMp

    BuMp Maha Guru

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    Thanx GL, u provided with some handy info.
     
  18. Dr. Vodka

    Dr. Vodka Ancient Guru

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    I cant even understand that guide, Man I speak spanish and a lil english not chinese!!! :D
     
  19. perera

    perera Active Member

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    its not complicated just think a bit =P

    2 points
    : = divide
    its just OC CLOCK : STOCK CLOCK
    OC VOLTAGE : STOCK CLOCK
    normal wattage multiplicated by result of clock :and stock clock. then multiplicate that result for the result of oc voltage : stock clock
    gets like this :

    (example)

    3400:3000 = 1.133

    1.45:1.35 = 1.074

    (stock wattage) 89 X 1.133 = 100.57

    therefore -

    100.57 X 1.074 = 108



    what id like to know is if fsb speed also diferenciate the watts consumage
     
  20. G L

    G L Don Juan

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    The equation is a bit easier to handle in the original article, about 2/3 of the way down the page here:

    http://www.pcper.com/article.php?aid=-105&type=expert&pid=5

    Just substitute TDP for "MaximumListedPower".

    The only problem is that I no longer trust AMD and Intel's TDP numbers. It used to be that each processor speed grade had its own TDP, now you just see one for the whole line... and I really don't see how that could be possible. I thought perhaps SOI or something about the Athlon 64 might explain it originally, but then Intel started doing the same exact thing.

    So for instance the 89 watts for the .13 micron Athlon 64 does allow you to do the equation, but unless you have the fastest processor for that TDP it presumbably results in too big a number. Of course, you can always knock off a few watts for each speed grade and approximate...

    ----
    Oh yeah, its actually (1.45/1.35)^2 = (1.074)^2 = 1.15... more like 116 watts, I think.
     

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