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Power consumption

  1. Dec 3, 2007 #1
    This isn't exactly a physics question, more of a household debate/puzzle:

    You get a laptop and it comes from the factory with the battery fully charged. It takes 1 hour of being plugged in (with the computer on and in use) to charge, and gives 3 hours of use (when unplugged, of course). Let's assume the computer is used for a total of 8 hours a day.

    Now, do you save energy by unplugging the computer, and using battery power for 3 hours, and the plugging it back in for an hour, and then unplugging it, etc.. or is the power consumption the same if you keep the computer plugged in for all 8 hours.

    (Note: That means that by the end of the day, the computer's battery is fully charged, so every day is the same.)
  2. jcsd
  3. Dec 3, 2007 #2
    You use more power unplugging it.

    The power the laptop uses is constant.

    The battery power is constant.

    But charging the battery is not perfectly efficient, so if you are charging the battery and using the laptop you use more power than when you use the laptop plugged in with a full charge.

    The charge/dischage circuits in a laptop are not as efficient as the ones for converting line power (switched A/C power to D/C vs. DC to switched A/C to Stepped A/C to D/C).

    Bottom line, using the Laptop plugged in, with a good battery and a healthy charging circuit is always the most efficient power usage. Occams Razor at home work and play.
  4. Dec 4, 2007 #3
    What's more, the plug in plug out will reduce the life span of the battery so by that way, you consume a lot more energy in manufacturing new batteries.
  5. Dec 4, 2007 #4


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    Staff: Mentor

    There is one caveat here in that a laptop likely has power-saving features that make it consume less power on battery than on wall power. But you can tell it to operate in that mode all the time.
  6. Dec 4, 2007 #5


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    You're far better off as said above, using a power plan. Set it to go into S3 suspend within minutes and keep it on mains power. It'll use very little power when in that state (barely enough to keep the RAM contents). If you can handle the added waiting, then setting it to hibernate would be ideal.

    That's assuming your laptop exists.
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