In the last decade researchers pursued the development of tiny flying robots inspired by natural flyers. Within this context our main objective is to devise a flying robot-mimicking insect in terms of kinematics and scale using MEMS technologies. For this purpose, an original design has been developed around resonant thorax and wings by the way of an indirect actuation and a concise transmission. Using such a design prototypes with a wingspan of 3 cm and a mass of 22 mg able to lift 75% of their weights are achieved. As a part of this global goal, this paper presents an innovative power modeling of a flapping-wing nano air vehicle (FWNAV) actuation-transmission system, aiming at a better comprehension of the power transmission from power source to wings, thus allowing future optimization of the actuation efficiency. Considering the multiphysics nature of the prototype a mechatronic approach has been chosen bringing thus a unique model for the whole system. This model has been realized thanks to Bond Graph formalism, which has the crucial advantage to be adapted to multiphysics systems and energetic analysis. After an experimental validation, it is demonstrated that the main parameter conditioning the overall efficiency of the FWNAV is the interaction between coil's current and magnet remanent magnet flux density.