Dim screen when on battery (i.e. when AC is disconnected).
For most laptops and use-cases the screen-backlight is the number biggest consumer of energy. Therefore when switching from AC to battery, it makes sense to reduce the screen brightness. I personally keep repeating the following steps: when undock (unplug) the computer, manually reduce the screen brightness; when docking (attach power), increase the screen brightness again.
Windows seems to be having two different "save points"/"settings" for the brightness, one for AC, one for battery. macOS only does dim the screen slightly and there is a settings to turn it of.
Had a chat with @aday about it and something like the latter is probably more what we want.