I have a 13" MacBook Pro, and battery life with Big Sur is noticeably worse on Big Sur. Hoping that an update settles battery usage down in the near future. I'd probably only be able to get about 3-4 hours of Screen On Time if I didn't charge periodically throughout the day.
I've found that if I am not running the built-in apps (Music, Messages, Podcasts...) my battery life is better. I think those apps are built on the Catalyst framework and it is not very efficent.