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I primarily use my Mac Book with the lid closed and connected to external keyboard, monitor and mouse. To save a small amount on the power bill, I would like to run it from the battery during the day and charge the battery at night when electricity is cheaper. I want to continue using the laptop regardless of whether it is switching from battery to mains or from mains to battery.

Currently, whenever I turn the mains off, the MacBook goes to sleep and I cannot wake it with the external wired mouse and keyboard.

I have set the power settings to "never sleep" wherever I can see it and have enabled "Prevent your Mac from automatically sleeping when the display is off", but still it sleeps when I turn the power off. Clearly, I do want it to sleep when I am inactive, but I have just been trying everything!

All other (non-Apple) laptops I have had will simply switch to battery when they lose mains power, without interrupting you.

Questions

With the MacBook lid closed, how do you,

  • wake the MacBook when it is on battery power?
  • keep the MacBook on when the power source changes?
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  • What model of Mac laptop is this? (e.g. year of model, Pro, Air, etc.)
    – benwiggy
    Commented Sep 18, 2022 at 16:57
  • MBP Running 12.5.1
    – froxpotatoes
    Commented Sep 18, 2022 at 19:58
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    A MacBook Air under "normal" wireless web browsing draws ~3.5 watts. In the UK where the average rate appears to be 36p/kWh, the hourly cost of 0.00126£/hour. If you use the laptop 18 hours a day then you would incur an annual cost of £8.28.
    – Ezekiel
    Commented Sep 18, 2022 at 20:17
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    The strain of draining your battery every day and recharging it (instead of keeping it plugged in) would likely devalue your computer more than the entire amount of energy you're using to run the machine. A new battery costs $129 (convert appropriately)
    – Ezekiel
    Commented Sep 18, 2022 at 20:18
  • Seems like there is an ongoing debate as to whether one should leave a laptop plugged in. makeuseof.com/tag/leave-laptop-plugged-time As you say, it is probably not worth worrying about though.
    – froxpotatoes
    Commented Sep 19, 2022 at 8:39

1 Answer 1

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Apple laptops have to be plugged in to work in 'clamshell' mode (closed, with an external display).
https://support.apple.com/en-gb/HT202351

If you want to run it off the battery, you'll have to open the lid. I'm not sure why Mac laptops are like this, but I'm sure there's a reason.

I'm not convinced that there is a saving to be made by running the battery by day and charging over night, versus leaving it plugged in all the time. Note that it's not actually charging all the time that it's plugged in. Fast-charging from a low-percentage is likely to use much more energy overall.

Apple laptops vary in energy storage between around 50 Watt-hour and 100 Wh batteries. Divide that by the hours of usage, and the rate of power use is well under 10W. Chargers vary between 30W and 140W (though not all of this is used for charging). You can use low-wattage chargers to reduce the maximum power draw, which might reduce your footprint a tiny bit.

It will also depend on whether you actually pay less at night for your electricity, of course. Depending on how much you use the laptop, you may run the battery down before the off-peak rate kicks in anyway.

The most efficient way to use your laptop in clamshell mode with an external display is to enable sleep, and let the laptop (and display) go into sleep mode when not in use.

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