Piezoelectric energy harvesters are popular systems for converting mechanical energy into electrical energy. According to the literature, there are many papers where authors try to improve the efficiency of energy conversion through modifications to the electrical or mechanical part of the system. This work focuses on the mechanical part of the harvester. This paper continues a line of work published by the authors in journals over the last three years. In this work, the authors show the influence of material and geometrical parameters on the efficiency of a beam piezoelectric energy harvester. The considered system is a three-layered cantilever beam (generator – core – generator), where the layers are perfectly bonded to each other. The length of the piezoelectric elements is equal to or shorter than the core length. The total thickness is assumed constant, while the thickness of each layer varies according to an assumed relationship. The Young’s moduli of the layers are related by a simple relationship. The harvester excitation is realized by the harmonic support movement with constant amplitude. The authors propose two models: an analytical model and a finite-element model. The analytical model is based on the Euler–Bernoulli hypothesis and the so-called effective electromechanical coupling coefficient. The FE model is two-dimensional with higher-order elements. In the first step, the results of both methods were compared with each other and shown graphically. In the second part, only FE-model calculations are performed, because they describe the considered problem better. The results for the chosen material and geometrical parameter values were compared graphically and discussed.