I left mine running over night with KOReader open, Nextcloud in background, no suspend and it took 20 hours and 10 minutes to go from 100% to 10%.
As @poweruser@lemmy.sdf.org wrote, there is a pretty significant phantom drain where it loses about 15% per day when suspended.
They have since fixed that.
Unfortunately just the nature of the technology. If you're just reading, DU4:3 works the best, for manga I use the full G:4 mode with screen refresh for every page flip enabled.
I wrote some custom profiles for each (Default, Book, Manga, Notes, Notes (Landscape) which I have on my desktop, I can send you the scripts if you want.
Couldn't find a good way to use browsers on it yet since they all smooth scroll instead of jumping in fixed intervals.