iPhone 7plus battery drains quickly after iOS 15 update

Try a Forced Reset. It's basically a computer so in case of doubt, REBOOT! Apart from that, it is not unusual to see a slightly higher battery load for a few days after a major software upgrade, while the phone does extra work in the background to reindex all the stored data.

If it doesn't improve or the battery life is significantly shorter it may be that your older battery is starting to degrade. Check Settings/Battery/Battery Health, and arrange to get battery service if it is below 80% capacity.

You could also have out of date or incompatible apps. NextDoor for example. or VPN.

• Uninstall or offload all apps you haven't used for a while.
• Make sure what remains is updated.
• Eliminate bad apps using the battery usage chart in Settings, Battery.

High battery drain is normal after a major update because there are a lot of changes being made even after the first time your phone starts again, such as internal data movement and app updates, etc.

This happens in background so your phone can be used as soon as possible. The downside is that it can take a few hours or a few days to finish which causes the battery to constantly drain until that's done - this should go away in a few days.

One additional comment, iOS15.1 software update has just been released. I suggest it would be worth updating your device(s) ASAP, because it might bring some battery improvements. It certainly won't do any harm to get the update before you spend time chasing other problems