Dear Farshad,
As I explain in my post dated Feb 24, 2017 in the following forum topic: Pitch rate in Fast v7 - #13 by Jason.Jonkman, RotAeroPwr (and associated RtAeroCp) output from AeroDyn v15 within FAST v8 are derived from the applied aerodynamic loads and RotPwr output from ElastoDyn within FAST v8 (and associated RotCp from FAST v7) are derived from both the applied aerodynamic loads and inertial effects.
By nature of their calculation within the aero-elastic solution, I would only use RotAeroCp and RotCp in a time-averaged sense (RotCp must be time-averaged longer than RotAeroCp) to avoid apparent violations in the Betz limit that may occur instantaneously as a result of the calculation approach.
Best regards,