Got the same problem 6 month after your post! Just made the upgrade to BIG SUR (11.2.3) yesterday and my fans go crazy, the battery goes empty in 1h on my MacBook Pro 15" from last year. The apple phone call can't help me after trying a few things like restart the mac doing a few things. They ask me to erase my HD and install everything again properly... but I'm not happy with this solution! So I tried to schedule the wake up as described by an other user and it works well! I hope this bug will be deleted in the next upgrade!!!