The base spec says “Max battery life: MobileMark 2014: 18.1 hrs Battery life varies significantly with settings, usage, & other factors”
Those sorts of statements always seem … optimistic. I’d say runtime is decent. The below are a set of estimates gathered during admittedly casual testing. Brightness is about 2/3 for all of these. I let each situation persist long enough that the “gas gauge” had stabilized.
(And no, I don’t know who The Posies are…)