Battery life

To understand Apple Watch performance and its relation to your battery, see:

Apple Watch - Battery - Apple

Apple Watch Battery and Performance - Apple Support

“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:

Batteries - Maximizing Performance - Apple

If the battery drains too fast after an update, see all the replies to:

Battery Drain since watchos 7.4 - Apple Community

WatchOS 7.5 battery drain - Apple Community

FWIW, in some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the battery draining issue after an update. (Simply unpairing and pairing again may not work.)

Unpair and erase your Apple Watch - Apple Support

Also, some people have found that simply waiting a few days after an update resolves the battery draining problem.

Apple says an Apple Watch 8 can get UP TO 18 hours in a days use. And that will vary based on what you do on your watch throughout the day. Speaking on the phone, texting, streaming music, reading and responding to emails, activities, etc., will use the battery faster. Unless you don't touch the watch during the time it's on your wrist, 15 hours isn't unusual.

This link may help you better understand --> Apple Watch - Battery - Apple