A composite piezoelectric energy converter intended for Micro-electromechanical Systems (MEMS) from background vibrations is presented. The converter is composed of a piezoelectric circular plate bonded to a brass substrate with different diameters. The vibration of the structure is analyzed based on the thermal-piezoelectric-elastic theory and Kirchhoff’s assumption. The vibration solutions and the relation between the vibration and output charge are obtained. The effects of geometric characteristics and environment temperatures on the electrical energy generation are numerically discussed. The numerical results show that the vibration-induced voltage is proportional to the excitation frequency and the thickness of the device, but is inversely proportional to the temperature of the environment. The experimental data show good agreement with the energy conversion analytical model.