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Charging a computer: power adapter output

  1. Feb 7, 2012 #1
    Hello Forum,

    I have a MacPro Apple computer. I just got a new power adapter rated 85 W.
    Does it mean that the computer is always drawing 85 W of power does is the power delivered depend on how many programs are open, etc...?

  2. jcsd
  3. Feb 7, 2012 #2


    Staff: Mentor

  4. Feb 7, 2012 #3
    I see, thank you.

    this one was given to me at the Apple store. I guess I was wondering if it continuosly have to draw the amount of power written on the adapter or it depends on the computer activities...
  5. Feb 7, 2012 #4


    Staff: Mentor

    the key parts in a computer the cpu/memory, the disk and the screen with backlighting.

    The disk probably draws the most power so constant disk access will drain your battery faster.

    Next the lit up screen followed by cpu/memory.

    Some cpus can change speeds depending on the processing requirements so a faster speed chews up more power, generates more heat and may cause a fan that also draws power to run.

    The most power saving mode is to turn the laptop off and use a piece of paper and pencil. :-)
  6. Feb 7, 2012 #5


    User Avatar
    Science Advisor

    In general adapters are designed to handle the maximum plus a safety margin.
  7. Feb 7, 2012 #6
    Ok thanks. I guess we found the answer: if the adapter is rated 85 W, and 1 KWh of energy costs around 15 cents, it does not mean that in 1 hour

    85 x 1 hour รท 1000 x 10 cents per kWh = .850 cents ?

    Ok that sounds like nothing, not even a cent? And we are saying that the computer may not even be operating in such a way to be always requiring 85 W, sometimes less...

    They say power is expensive? Where is my mistake?

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