Can I put battery level on watch face?

You can. I use a 3rd party app called BatteryPhone. It has the charge level of both your watch and your phone. It's in the Watch App Store.

ASC - Late 2012 Mac mini, macOS 10.15.6; Watch, series 5 GPS/cellular, watchOS 7; iPad Air 2, ipadOS 14; iPhone 8+, iOS 14; TV 4th Gen, tvOS 14  HomePods, audioOS 14 and Airport Express w/ Airplay 2