Measuring the battery’s fullness is basically free. You have to measure the slowly dropping voltage coming from the battery (which is a proxy for its remaining power) to compensate for it anyways. This reddit thread explains it nicely: https://www.reddit.com/r/explainlikeimfive/comments/1qrdc9/eli5how_do_digital_devices_know_how_much_energy/
Having the screen on would be the biggest drain here. They’re trying to minimize drain for folks with OLED screens by using a solid black background, but that doesn’t help for any other screen type.
