The faceID technology is significantly more complex technology than the touchID. it’s basically a Microsoft Kinect crunched into that small notch. Think about that for a sec.Yet the plus has a smaller battery and supposedly a less power-efficient and slightly larger display.
My 8Plus right now is sitting at 76% with 4hr 24minutes of usage and 11hr 29 minutes of standby. I easily get 8-10hours of usage a day, 18+ hours of standby, and that includes at least 1 hour of Pokemon Go.