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Using iStat Menus, I tested the power usage of my MacBook Pro when idle and plugged in. When it was off the charger, it used around 5-6 W of power consistently. When I plugged it in, however, it used 6-7 W of power consistently. I repeated this multiple times over multiple days, and in general it uses more power for any task (or even when idling) when it is plugged in.

Why does the Mac use slightly more power when plugged in, when seemingly doing the same thing both times? Does the Mac underclock itself when it's running on battery power? Would underclocking still have an effect on power consumption is the laptop is idle?

Note: I had "dim display on battery" disabled, so it shouldn't be due to a change in monitor brightness.

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When you disregard things such as dimming the display on battery and CPU-throttling - you're left with a quite obvious difference: charging the battery.

Even at 100% charge, you'll spend some extra energy keeping the battery charged while using the computer.

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  • "obvious difference" being an understatement!
    – Allan
    Jul 27, 2020 at 22:59
  • Interesting. Didn't know it would be on the magnitude of 1W. In iStat Menus, the battery current seems to be 0.25W. However, this may be within the margin or error
    – Oion Akif
    Jul 28, 2020 at 21:02

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