Okay, so it's been 7+ hours with "tap to wake" and I'm still at 80%. Yesterday, with the watch "as is" out of the box, I'd lost 35% in 3-4 hours.
Besides changing to "tap to wake"... one other change I made late yesterday - which someone else tried to point out earlier in this thread - is disable background app refresh for most of the apps. For whatever reason, they were all turned on after I migrated from my old watch to the Series 5. Now I've turned it off for everything except Activity, Alarms, Calendar, Duo, Glympse, Reminders, and Workout. The watch face is Modular Compact (Analog) with Date, Weather/Temp, and Calendar.
Based on this extremely small sample set of exactly one watch, I'm going to go out on a limb and blame the problems many of us saw on the new complications (like Noise), plus the usual battery issues with a major WatchOS upgrade.