Sorry if this has been talked about already, I couldn't find anything. I have moved from garmin where they have body battery and this is the closest I can find on apple watch ultra. I'm currently on the trial and so far I think it's great. My only question is no matter what my level was before bed it always seems to start the day at 100% despite how well I've slept or how low it was the day before.
With garmin if I'd had a bad nights sleep or the day before was very physical it would start the day at a value representative of those parameters. On Saturday I stayed in a tent after spending all day Saturday pushing a bike up a hill so was pretty tired on Sunday morning but it showed 100%, garmin would have showed about 60-70% based on previous situations. So I think the 100% start is not a good indicator of how much energy I had for the day ahead.