party_dot If there are two identical Pixels with the same settings and set of apps, which one is expected to have longer battery life, the one with GrapheneOS or stock Android?
It depends on the apps.
For example, I believe recently it was discovered that OsmAnd was running more slowly on GrapheneOS than on the stock OS, and there was some reason to believe that the app was very frequently allocating and freeing memory, so disabling the hardened memory allocator for that app made it run a lot faster. Because the GrapheneOS memory allocator includes lots of exploit countermeasures, using it very heavily will cost more energy.
In the other direction, on GrapheneOS because Google Play isn't privileged it can't do whatever it wants to whenever it wants to do it, so it may run less. Also, GrapheneOS pre-compiles apps from Java to machine code, whereas the stock OS relies more on JIT compilation of Java.
Different users report different experiences.