iOS 14.7 Battery Drain

I'm not ruling out a battery drain problem, but you can't judge battery drain related to a new iOS version within a day of the installation. Every update has some housekeeping to do (varies by update) which takes a lot of cpu time and thus a lot of battery draw, and generates heat. Variables include what needs to be done and how much stuff is on your phone. For example, an update with only some security fixes may only need a few minutes to do its after update chores, while the update a few years ago that changed how each photo was stored and indexed took many hours on a phone with a lot of photos.

For this 14.7 update, on my old iPhone 7 with little on it, it used close to 100% cpu time for several hours and got quite warm. But the next day it was fine. My iPhone 11 went quite a bit faster and also was fine the next day. I didn't try to run from the battery or draw any conclusions about battery draw (or performance, since a lot of cpu usage elsewhere can make the phone feel sluggish) until at least the next day.

When I do updates, I take the case off so heat can dissipate better, insure the battery is fully charged, back up the phone, reboot the phone, then with the phone plugged into a charging block, do the update and then leave it connected to the charger until the phone has cooled down to room temperature for at least a few hours, and then keep it on the charger as much as I can for the next 24 hours.