That sounds a LOT higher than I’d expect, and a lot higher than pretty much every reported usage I’ve seen for the first-gen iPad Pros (both the 9.7-inch and the 12.9-inch) on iPadOS 16.
It’s not that I don’t believe you, but I’ve seen that data read incorrectly thousands of times; are you counting since you unplugged the iPad? If you’re counting from last 10 days, you might be including screen-on time from the previous cycle. Like I said, I’m sorry to be skeptical, but I’ve seen thousands of screenshots incorrectly reported, and it really sounds too high.
I’m barely scraping 10-11 hours of similar use (web browsing and media consumption), on my 9.7-inch iPad Pro on iOS 12, with low brightness. That use is light, and I’ve never been able to get any more... on iOS 12. 9 hours would be at most two hours below what I’m getting, and according to everything I’ve read, it’s too much. I’ve optimized every single setting, and I know how to do it: I’m getting 16 hours of screen-on time on my iPhone Xʀ. I’ve only seen one other person, ever, get 16 hours on a Xʀ. The vast majority were at 10-11, at best (on iOS 12, like me). On iOS 16? They’re barely scraping 6.
I’d expect 6-7 with light use maybe, at best? I’ve seen some screenshots and they were all on about 4, but with lighter usage, 6 or 7 may be possible.
I’m not discounting the possibility of me being completely and entirely wrong (I’d never do that): if I am, that’s great news for all first-gen iPad Pro owners, at least battery life isn’t as obliterated as it could be; in fact, I hope I’m wrong.
For what it’s worth, I get better battery life from the 2017 12.9 compared to the 2017 10.5 (using both at 0-20% brightness) even when both were new. The 12.9’s battery hasn’t degraded as much as the 10.5’s either even with fairly heavy use. The 12.9 was used for ~8-9 hours/day at the office on weekdays for email, instant messaging, web browsing/research, PDF annotation, etc. while the 10.5 was my e-reader, web browser when I get back home.
The 12.9 has a 41Wh battery while the 10.5 only has 30.4Wh. I’m sure the bigger battery is required to power the larger, higher resolution display. It may not need the 30% higher capacity for that, though, particularly if one is using it at low brightness (iirc, power consumption increases logarithmically/exponentially with increased brightness).
Mind, the iPad 4th gen has a ~40Wh battery as well and that one lasted forever.
Probably same situation as iPhones. My 4.7” iPhone batteries have degraded fairly quickly despite light use (like 10 minutes per day average onscreen time). Meanwhile, my dad’s and brother’s 5.5-6.1” iPhone batteries maintain good battery life for quite a long time despite being used far more heavily.