tracking the pwr capacity available

Posted on 2005-04-11
Last Modified: 2010-04-26
I there a way to find out how much of the wattage on a power supply is used up? I have a pheonix bios and I am going to be adding extra hard drives and a cdrom.
Question by:peds72
    LVL 95

    Expert Comment

    by:Lee W, MVP
    Nothing really built in, you can get yourself a "KILL-A-WATT" meter for $25-$40 on ebay - I have one and love it.

    For example:
    LVL 7

    Assisted Solution

    hi there
    you can calculate the overall requirements
    the following is from

    Accelerated Graphics Port (AGP) card 20 to 30W
    Peripheral Component Interconnect (PCI) card 5W
    small computer system interface (SCSI) PCI card 20 to 25W
    floppy disk drive 5W
    network interface card 4W
    50X CD-ROM drive 10 to 25W
    RAM 10W per 128M
    5200 RPM Integrated Drive Electronics (IDE) hard disk drive 5 to 11W
    7200 RPM IDE hard disk drive 5 to 15W
    Motherboard (without CPU or RAM) 20 to 30W
    550 MHz Pentium III 30W
    733 MHz Pentium III 23.5W
    300 MHz Celeron 18W
    600 MHz Athlon 45W

    hope this helps
    LVL 14

    Expert Comment

    Careful attention to the power rails...T
    LVL 13

    Accepted Solution


    If you have a 350 watt power supply, that is the total OUTPUT power.  That power exists on a number of "rails", or voltages.  There is a 5 volt rail, a 3.3 volt rail, a +12 volt rail and a few others.  To determine the power being used, you would have to measure the current drawn by each rail, multiply it by the voltage for that rail, and add them all together.  Unfortunately, however, you cannot measure current without cutting wires or using some very expensive test equipment that you dont have.

    But there are some problem:

    Catch #1:  You can hit the "limit" on one rail before you hit the limit on the overall power supply.  So if the limit on the 12 volt supply is 15 amps, you could hit the limit on the 12 volt supply (which is limited to only 180 watts) WAY before the total power supply limit of, say, 350 watts was hit.

    Catch #2:  You can't measure instantaneous peaks or the power supply's ability to respond to them.  While the total power requirement for both the power supply and all of the individual rails may well be below their ratings, the CPU and graphics cards may have tremendous instantaneoud peaks FAR above the continuous ratings of ANY power supply (a CPU can draw as much as 80 amps or more .... for a couple of microseconds).  The ability of a power supply to meet these falls outside of the normal ratings of the power supply, and is dependent on the size of the capacitors in the supply.  There is no way for us end-users and even techs to measure this, although it can be done in a lab.  But it's a major difference between a "good" power supply and a "cheap" power supply.

    Catch #3:  I also have and love the "kill-a-watt" meter.  But keep in mind that it measures input power, not output power.  The conversion efficiency of any given power supply is unknows, but is typically 60% to 85% or so.  So a 350 watt power supply, that is actually putting out 350 watts, will likely be drawing anywhere from 412 watts to 584 watts from the wall (which is what the "kill-a-watt" will measure).

    Catch #4:  The ratings may not mean much anyway.  I'll take a high-quality 350 watt supply over a cheap $20 wonder 500-watt supply any day.

    The bottom line here is that the best place to start is with the kind of estimate that Philby suggests.  But since that's only an estimate, perhaps the best approach is pragmatic:  If the system is running ok and is stable, especially if it meets this test even when running a "stress test" that draws maximum CPU and GPU power, then the power supply is meeting the needs of the system.  If the system is unstable, if it locks up or crashes or blue screens, then the power supply, ratings aside, should almost always be considered as a potential culprit.

    Author Comment

    Thanks guys that info was helpful. I knew nothing about rails and the estimates of pwr usage from the website will serve as a good guide. Thanks.

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