Before, iOS would show you how long the phone had been active — which included something running in the background with the screen off. Now they show "Screen On" time, so you'll show less "time" between charges than you did previously. From what I've seen, battery life should be similar to iOS 11.

If you are getting less battery life, then I'd suggest rebooting the phone. Sometimes something "hangs" in the final part of the upgrade process, and a reboot will clear the issue. Of course, if that doesn't work and you are getting much less battery life, do a full reset of the phone. Ideally, it works best if you set up as new, but often you can restore the phone and fix the issue.