If you’re using Dark Mode under high ambient lighting the difference can be astronomical. Check this out from DisplayMate’s testing. “An additional bonus is that switching to a Black Background will in most cases significantly reduce the battery power used by the OLED display. For example, for the Calendar application, at the Maximum Brightness setting, switching to the Dark Mode from the Light Mode reduces the Display Power from 4.2 watts down to 0.2 watts, which will significantly increase the Running Time on Battery.”