iPhone 17 Pro Max battery draining faster than normal on iOS 26.0.1

Leave your device on at night for a few days and let the battery charge to 100%. It should improve a little once the indexing and syncing complete. For example I had to erase and reinstall almost a week ago and my battery is acting similar to yours. Your device is too new to determine anything. Wait until about 10,days old and let’s take a look.

You should be charging your phone all night every night and I recommend using the Optimized Charge setting. If you are using a charge limit, you are already starting each day with a partially depleted battery. Your phone under any iOS version is going to drop several percent overnight while idle. What you described is having all day battery life and exactly what you want. Don't get caught up comparing Screen on/off time as you can see adding the 2 together never equals 24 hours which are the only 2 states other than when your device is powered off.

Not sure how many days ago you set up your device, but as Jeff Donald said indexing and syncing will continue for several days. This causes battery drain, lagging, and increased heat as seen by the Support Article Apple has provided. Give it the 10 days like has been recommended.

About Apple software updates - Apple Support

That looks normal. If you charge overnight with Optimized Charging enabled (NOT 80%) your phone should not need charging during the day, which is the design goal for iPhones.

Also, from→About Apple software updates - Apple Support:

Immediately after completing an update, particularly a major release, you might notice a temporary impact on battery life and thermal performance. This is normal, as your device needs time to complete the setup process in the background, including indexing data and files for search, downloading new assets, and updating apps.

What happens when you charge to 100%? Are you leaving your iPhone plugged in at night and turned on?