Brightness is quite low. Don’t really want to hit low power mode
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Safari-heavy use on an original version of iOS with 6h on 50% which translates to about 11.5 hours of screen-on time with fairly low brightness definitely isn’t bad.
The larger 12.9-inch screen may be a little more draining than the 11 or 9.7-inch screens that I’ve used. My 9.7-inch iPad Pro before it was forced out of iOS 9 gave me about 13-14 hours of Safari-centric use, and my iPad Air 5 seems to be at like 11-12 hours with 50% remaining, which is absurdly high, but with extremely light settings. The larger screen and non-perfect settings make a 12-hour screen-on time quite decent.
My 9.7-inch iPad Pro saw a decrease to about 10-11 hours with heavily optimised settings and light usage after being forced into iOS 12. I’m only guessing, but it might get about 8 hours with less optimised settings and higher brightness, so your 12 hours seem good.