In this paper an effective model for the power and efficiency analysis of Wireless Power Transfer Systems (WPTSs) is proposed. Such enhanced model includes modulation of duty-cycle and phase-shift for the secondary side controlled rectifier, as well as power losses of semiconductor devices and parameters mismatches of resonant elements. The global influence of semiconductor devices parameters and passive elements on the overall WPTS performances is numerically determined by solving the non-linear equations of the system discussed in this paper. Simulations and experimental measurements referred to a 2 W@6.78 MHz WPTS fully demonstrate the validity of the proposed analysis