This paper presents a model for energy transfer between piezoelectric wafer active sensor (PWAS) and excited structures. Analytical models are developed for PWAS admittance, active power, Lamb waves kinetic, potential energies, and power. This model is based on Lamb waves; accordingly it is valid for high frequency multi-modal excitation analysis. Numerical simulation is done for (a) thin aluminum plate, where only first symmetric and antisymmetric exist, and (b) thick steel plate example is presented to show the case of multi-modal wave propagation. Parametric studies for different excitation frequencies and different transducer sizes are done to show the best match of (frequency/PWAS size) for exciting structures.