https://www.techradar.com/news/iphone-xs-iphone-xs-max-and-iphone-xr-battery-sizes-have-been-revealed

If you follow Apple announcements, you'll likely know that the company never unveils the battery sizes of its phones on stage or after the press conference... but we now know the capacity of the batteries inside each of the three new iPhones. 

The information comes from a certification body in China, which published documents confirming the exact specifications of the battery packs and which was found by MyDrivers.