Poor Battery Life
Every major iOS upgrade is accompanied by users reporting problems with battery life, but this is actually one of these things that’s perfectly normal — at least for the first few days after you upgrade.
Since Apple would prefer to get you back up and using your iPhone as quickly as possible, it leaves a lot of stuff to finish up in the background after a major iOS upgrade, in the same way that booting up a Windows PC or even a Mac presents a user interface right away, but can still be sluggish until everything has finished loading.
In the case of iOS, this includes things like battery recalibration, Spotlight indexing, and photo curation, among other things. Generally, however, this should settle down after a few days.
If you’re still having problems after that, it’s certainly possible that you may have run into an iOS 15 bug, but it’s also equally likely that one of your third-party apps is misbehaving in some way. In fact, we’ve already encountered one popular app that’s a major culprit behind battery drain problems in iOS 15 (and iOS 14.8 too), and it’s fair to say there could be many others.
In this case, turning off Background App Refresh in the General section of your iPhone Settings app can help to mitigate the problem.