When you upgrade to a new iOS version, the backup is restored and photos/videos are something that takes a lot of time to download completely. Because the iPhone is being overworked in the background, the battery drain is faster. It is advisable that you put the iPhone on charging and it will help in saving battery life.
Check your Photos app
There’s also a new iOS 16 feature in the Photos app that deals with all the duplicate photos in the photo library. With iOS 16, Apple has given a feature where the iPhone automatically goes through the entire library to identify duplicate photos. If you have thousands — and many people actually have more — of photos on your iPhone, the duplication process takes a lot of time and hence ends up consuming a lot of battery.
Update your apps
After a software update, apps need to be in sync with all the new tech that is introduced with iOS 16. Developers have been updating their apps to support the new features and tech in iOS 16. However, users might have not updated them. If the apps haven’t been updated, then they could drain more battery than expected.