This paper reports the experimental and numerical study of flexible piezoelectric transducers made of a thin active composite material (~3 μm thick) over a thin flexible metallic foil (~25 μm of stainless steel). The active piezo-layer consists of vertical ZnO nanowires (NWs) embedded into dielectric fillers of different composition. The voltage over a known resistance was measured with an automatic bending setup, and the corresponding power was computed and compared to numerical simulations as a function of load resistance and dielectric matrix in the composite structure. FEM simulations show that the output power can be higher than conventional thin-film devices. These results confirm previous findings and provide important guidelines to optimize flexible piezoelectric transducers for applications as sensors and generators for the Internet of Things. Comparison with a commercial thick-film piezoelectric energy harvester was also made experimentally.