We present a weakly non-linear model of a piezoelctric flag immersed in an axial flow. Numerical simulations based on Galerkin projections are carried out in order to evaluate the coupling between the flapping piezo-electric flag and a purely resistive circuit intended to simulate the electric energy harvesting circuit. The numerical simulations allow to point out the relevant physical parameters governing the apparition of the flapping instability, power conversion and system’s harvesting efficiency.