Piezoelectric energy harvesting from vibrating structures under nonlinear oscillations has been proved quite promising, with a large amount of scientific works focusing on the maximum energy harvested. However, the detailed dynamic response of an electromechanical coupled nonlinear vibrating system and the interaction between the piezoelectric structure and an external electric circuit deem to be an extremely important parameter of interpreting this complex dynamic phenomenon. Current work focuses on a combined experimental and computational campaign conducted to analyze and quantify various factors affecting the efficiency of energy harvesting in vibrating nonlinear piezoelectric–composite strips subjected to compressive loading. The model aims to simulate the power dissipated at an external purely resistive circuit connected at the piezoelectric device terminals during nonlinear oscillations. Experimental studies on a fabricated prototype are also conducted for different excitation frequencies and resistive loads, as to provide an estimation of the energy dissipated. In addition, a preliminary experimental study is performed to quantify the available electrical energy that is produced from the oscillating structure into three different harvesting circuits. Measured results exhibit their performance, indicate their benefits and drawbacks concluding to the most promising piezoelectric energy harvester.