Why is my Apple Watch Ultra battery draining quickly after updating to watchOS 11?

After an OS update, it's common to experience an increased battery drain as certain tasks related to the update continue in the background including indexing, you should only experience this increased battery drain for a few days before it goes back to normal.

If after a couple days, your battery is not back to normal, I would recommend unpair and repairing your Apple Watch with your iPhone ⇢ Unpair and erase your Apple Watch - Apple Support

Have done the add and remove. and no success the battery if I don't turn on battery saver will only last about 8 hrs a day. this watch is an apple watch 10 model. ive had improvment by turning of location and air drop to gain about another 30 minutes.

Had the same problem today. Only 8 hours and my battery died. It drained so quickly suddenly. It is also a new apple watch 10. It did well the first days but now it drained like ****. No apps in background etc….

No problem, have a wonderful day!