How Power Supplies Are Rated

Discussion in 'Power Supplies' started by jonnyGURU, Mar 19, 2008.

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  1. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    What is the "wattage" number (i.e. 650W, 800W, etc.) actually telling me?

    When you see the wattage rating of a power supply, you?re seeing the total maximum output capability of that particular power supply, but a computer has multiple voltage needs, and newer computers require more of the power supply?s capability to be on the +12V DC output rail. CPU?s and GPU?s regulate their power off of the +12V DC rail. Also, all of the computer?s motors run off of +12V DC: hard drive and optical drive motors, fan motors, pumps for water-cooling, etc. It wasn't too long ago that graphics cards did not require auxiliary +12V power and CPU's use to regulate their voltage from the +5V rail. An older power supply may have a lower percentage of it's power on the +12V than a more current unit.

    What is the difference between "continuous" and "peak" ratings?

    Some power supply units are rated for continuous output while others are rated at peak. "Continuous" means that the power supply is rated to run at it's maximum capability for no pre-determined period of time, while "Peak" indicates that the power supply will only run at the specified wattage for a brief period of time, possibly only a few seconds or up to a minute. This number is typically about 100W more than the power supply's actual continuous rating.

    How does the temperature inside of my case affect the performance of my power supply?

    Power supplies can perform differently depending on the temperature at which they are operating at. When a power supply is rated for it's total output wattage, it is rated to do so at a particular temperature. Anything beyond this temperature may take away from the power supply's capability. A power supply that is rated to put out 550W at 25°C or 30°C (room temperature) may only be able to put out 75% of that at 40°C or 50°C (actual operating temperature). This difference is called the "de-rating curve". A normal operating temperature for a power supply is 40°C.

    Is the temperature at which MTBF is measured at an indication of what temperature the power supply's output rating is measured at?

    Unfortunately, no. It's a tough race out there and there are a lot of guys rating their PSU's MTBF at room temperature, even if they rate their PSU at operating temperature. Fact of the matter is, MTBF can be a significantly, often exponentially, lower number when going from 25°C to 40°C. For example, one unit with an MTBF at 100,000 hours @ 25°C can have an MTBF of 20,000 hours at 40°C. That's a pretty big difference! So it's not unusual for a manufacturer to use the higher MTBF number at the lower temperature and, in most cases, not tell us at what temperature that MTBF is derived at. But even when they do tell us the MTBF temperature, this doesn't mean the PSU is rated at this. A PSU's output capability may not be seriously compromised by heat. If the PSU does 700W continuous @ 25°C and only 600W @ 40°C, the difference may not be significant enough to the manufacturer to increase their continuous wattage claims, so although they may measure MTBF at 25°C., they may very well be rating the PSU at 25, 40 or even 50°C. Unfortunately, it all comes down to marketing. It's easier to market a PSU that runs at what it's rated at 40°C then it is to market a significantly lower MTBF at the same temperature.
     
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  3. Sheninat0r

    Sheninat0r Senior member

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    Wow... an explosion of stickies in the PSU forum.

    Very informative and well written - good job jonny!
     
  4. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    It's important the CORRECT information is out there. Same questions keep getting asked over and over again and often the wrong information is given back in return.

    I have one more sticky to do, but I need to wait for my sticky rights to kick in. ;)
     
  5. bryanW1995

    bryanW1995 Lifer

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    the statement regarding temperatures is particularly relevant as you get into higher-quality psu's. You can get a 550W psu no-name brand for $30 or less, so why would you spend 2-3 times that for an hx520? For one thing, it's rated for continuous output at 50c. I used to buy the $24.99 specials on newegg, but after coming here realized that the psu is quite possibly the most important component in your entire system. well, that and my psu died and took the mobo with it last year :(
     
  6. pugh

    pugh Senior member

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    you know I'm a idiot. All this time I have heard of you but never knew you were a member here and a mod. I have been here since 2000 hehe. You and your site are a great help!
     
  7. Foxery

    Foxery Golden Member

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    Just to clarify, this means that a "500w" PSU is designed to give 500 watts of DC to the computer, and therefore draw ~600w AC from the wall?

    Might also be worth noting which components do still run on the 3V/5V rails. Hard drives, fans... what about PCI/PCIE cards?

    Any tips on where to find this part of the rating online? I don't see this listed anywhere obvious on, say, Antec's web site or the Detailed Specs section when buying from major retailers.
     
  8. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    Just to clarify, "500W", for example, means the PSU can put out, as in "DC Output", a total maximum of 500W regardless of how the load is distributed across the several voltages.

    Also, a PSU only delivers what's asked of it, so it's very unlikely that someone that is powering a PC that a 500W PSU is suitable for is actually getting 500W from the PSU. But if that PSU was delivering 500W and was 84% efficient at full load, it would be pulling 600W from the wall.

    Yes, of course computers still use +3.3V and +5V, etc. but not nearly as much as they used to (pre ATX12V and EPS12V) and far less than what virtually any PSU on the market is actually capable of producing.

    +12V is used to power CPU, GPU (video card) and is delivering this power via the slot and via auxillary power connectors. Motors, like drive motors, fan motors, etc., also use +12V. +5V powers most logic boards like hard drive PCB, USB devices, etc. +3.3V is typically for RAM, although sometimes a motherboard manufacturer may regulated RAM's voltage off of a +5V rail.

    If a PSU company has "nothing to hide" it can be found in documentation. For Antec, for example, it's on the side of the box. If it's not listed, it's anyone's guess. Nobody is "required" to disclose this information, just like nobody is required to disclose if a unit is rated at continuous vs. peak, etc.

    For the record, MTBF is NOT necessarily what a PSU is rated at. So a PSU can show an MTBF rating done at 25°C, but it's wattage could have been determined at 40°C.


     
  9. VulcanX

    VulcanX Member

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    Very interesting to see how the temps actually affect things, and with this in mind, i would like to know what MTBF is, or is that the total wattage along with the temp it can withstand, im a lil bit confused with that, any clarity to that?
    thanks in advance Jonny, you know your shit man! sorry for lang but very nice threads in your category
     
  10. HOOfan 1

    HOOfan 1 Platinum Member

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    MTBF

    Mean Time Before Failure

    It means the average amount of time you can expect the Power Supply to work until it loses its usefullness.
     
  11. Phaeded

    Phaeded Junior Member

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    I got into a debate with my father over which is better, to leave a system on contiously with low power settings, or turn it on and off daily as you use it. How does powering the system on and off affect the MTBF?

    Also - are some PSU's rated better at continous use over the on/off or vice versus?

     
  12. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    That's a good debate. Power cycles and components expanding and contracting from heating up and cooling down isn't good for ANY component. So it really depends on how long the PC is on and how long it's off. I would have to say that I would personally rather run the PC 12 hours on and 12 hours off than have it run 24 hours. BUT I would not want to shut down a PC for only 4 to 6 hours. Know what I mean?
     
  13. Phaeded

    Phaeded Junior Member

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    thanks for the advice.
    When a computer goes into hibernate, or sleep, or low power mode, etc etc. Does it effect how much energy is being pulled from the wall or just how much is being consumed by the PC? For example, would I notice a significant difference on my electrical bill if one month I left the computer on, and let it cycle into low power settings when not in use, and the next month, shut it down after each use, and the third month I shut it down and unplugged it?

     
  14. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    The amount the PC consumes directly effects how much is being pulled from the wall. A power supply only puts out what is demanded of it, so if the PC is in standby and drops from 200W to 5W of power consumption, the PSU is pulling less from the wall.
     
  15. ChrisScar

    ChrisScar Member

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    I have a DV9000 Pavilion. Occasionally, the adapter gets extremely hot and sometimes gives off a constant whirring noise. Does anyone know why this happens? It doesn't happen as often but annoying nonetheless.
     
  16. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    This has nothing to do with this thread. Start a new thread.
     
  17. NoShangriLa

    NoShangriLa Golden Member

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    This is my first time peruse through the PS forum, and I notice that the suggestion of derating is a little out.

    Using the PS calculation below a 550W at 60C would deliver 440W, and so on.

    According to the general Power Supply Derating Guideline:

     
  18. HOOfan 1

    HOOfan 1 Platinum Member

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    Read his description of derating closer...
    Different manufacturers rate their power supplies at different temperatures....the example he used was a manufacturer who rates their 550W PSU at 25C.

    Besides that, different PSUs can have different derating curves based on the components they use.
     
  19. NoShangriLa

    NoShangriLa Golden Member

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    From what I have seen in the past & electrical codes, electrical equipment aren't rated at room temperature. Early failure tend to be poor maintenance (dusty/dirty environment) or manufacture defect.

    I would like to see link/s to manufacture spec sheet that rate their equipment at room temperature or 25C.

    For example the AOpen AO450-12ARM is rated for 450W with operating temperature range of -20C to 65C stroang (I have seen PSU with stroang rated to 80C). It indicates that it will operate up to 65C and deliver the required 450W within their MTBF.

    It is likely that the derating curve varies from manufacture to manufacture, however the 50C operating temperature before generic derating is quite conservative. If, I remember correctly 20% derating per 10C also is a standard use in electrical codes for wirings, and electrical parts.

     
  20. OddJensen

    OddJensen Member

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    So say, to take an example, the PCP&C 750W is rated at 750W@40C. What would its new wattage be if it was to be re-rated (without any component changes) to 50C? ~625W?
     
  21. HOOfan 1

    HOOfan 1 Platinum Member

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    I've heard that Seasonic units like the PC P&C Silencer have a derating curve of 10W lost for ever 1C increas in temperature...at least above 40C.

    Which would mean it would re-rate to 650W at 50C. But on Seasonic's website They state that the basic platform of the Silencer will drop from 100% rating at 40C to 80% rating at 50C which would be 600W.

    Though at 25C Seasonic calls it the x900

    Considering it was easily able to do 750W at the 45C that HardOCP tests at and 746W at 51C for HardwareSecret's test...I think you can say the unit is a bit underrated.
     
  22. JASTECH

    JASTECH Senior member

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    I read that OCZ has some 90% effi. PSU's http://www.overclockersclub.com/news/24439/

    Time for a test Oklahoma Wolf?

    Jonny, that Enermax Rev 85+ you reviewed looks to be my next PSU in a month as my HIPER is going to my Wife in a CM 832 w/ 4 250GB drives and she will need it. Now, on my new system what watt would you suggest? I was thinking the 1050w Rev 85+ what do you think Oklahoma Wolf?

    Ga-EX58-UD5
    Intel i7 920
    Mushkin 6GB DDR3 1600 (3X6GB)
    ASUS Xonar DX
    NVIDIA GTX 275
    LSI SAS3081E-R
    2x LG GGW-H20L
    8x Hitachi HUS153073VLS300 73GB 15000 16MB 15K SAS Drives
    LIAN LI PC-V2110

    Thanks, JASTECH
     
  23. HOOfan 1

    HOOfan 1 Platinum Member

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    Jonny doesn't work or Jonnyguru.com anymore. Oklahoma Wolf does the reviews. BTW the Z1000 OCZ appears to be the same platform as the Hiper M1000 OK Wolf reviewed and the Tuniq Rippler Paul Johnson reviewed at HardOCP.
     
  24. JASTECH

    JASTECH Senior member

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    Hmmm, Ok. So some rebaging was done? Well then my question needs to be to Oklahoma Wolf, I shall edit my post. Thanks HOOfan 1
     
  25. HOOfan 1

    HOOfan 1 Platinum Member

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    It is made by Sirfa which is a spinoff of Sirtec. I am sure it is not exactly the same, as the Ripper and M1000 are not that efficient, but it looks similar.
     
  26. JEDIYoda

    JEDIYoda Lifer

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    No rebadging was done!!
    The definition of the word "rebadge' when it concerns PSU`s mean that the PSU that you say is rebadged uses exactly the same components and looks exactly the same as a PSU that has a different companys label.

    Just because it uses the same platform does not mean the PSU is rebadged!

    A good example of a rebadged PSU would be if you bought say brand A and then brand B....
    You took them both apart and they used exactly the same componemts and caps and heatsinks they looked exactly the same. mThat is 'rebadging".
     
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