Have you tried disabling animations?
I just upgraded a few days ago from Marshmallow, and have had battery life problems, with what seems like unusually high battery usage by Android System and Android OS -- about 2/3 as bad as the OP reports, and enough to make it difficult to get through a full day on one battery charge.
First I tried the suggestions of disabling Ambient Display and Adaptive Brightness, and revoking Body Sensor permissions from Google Play services as I found
suggested elsewhere, and still had trouble.
I tried clearing the cache (both the easy steps in Settings > Storage and clearing the partition from the device Recovery mode), but still had trouble.
Last night I disabled animations (enabled Developer Mode, changed all the animation scale settings to No Animations). With Ambient Display and Adaptive Brightness still disabled, it looks like I have much lower battery drain now. It's too early to declare victory, but I am finally encouraged that Nougat might fare better than Marshmallow. It's a bit of a shame, as the OS feels more harsh with no animations, but I can live with this. I hope I'll be able to re-enable Adaptive Brightness, as I do miss that when using my phone outdoors.
-Peter