Though to be fair if auto brightness is using power while the display is off someone needs to send Google developers a "software design for dummies" book...
(And if the display isn't off when the phone is idling then there is your problem
)
It will use the lux sensor to sense the surrounding light, whenever the screen is on.
If auto brightness is not used, then this sensor is (should be) not running.
Also, when the screen is on, the actual act of Auto Brightness adjusting the brightness up and down uses power.
Not to mention that now, on modern devices, this auto control also supposedly 'learns' your brightness habits.
Mine did not (after a week or so), and I was constantly battling the stupid auto crap in order to properly/comfortably see the screen.
It was crazy!
I would go outdoors into bright daylight, and the auto crap put the screen at ~80%.
I would pull down the quick settings, turn the brightness all the way up, just to see it go right back down.
Again and again.
Likewise in the opposite, it would hover around 70% to 90% indoors with a 40w equivalent bulb across the room.
Down to 25% I would place it, only to watch as it creeped right back up.
Stupid automatic junk.
It wasn't learning anything from me, unless I was telling it how to piss me off while I was talking in my sleep.
So I put an obsolete app called Brightness Control (from Curve Fish Apps) on it, and it gives me quick, pinpoint brightness control even faster than Auto Brightness could figure out where I did not want the brightness to be.
