Recent trends in electromechanical conversion using alternative materials, have demonstrated the advantages of using piezoelectric composites for energy generation. There have been several papers on ceramic/polymer composites in which the fillers have high piezoelectric constant. Basically, the energy harvested depends on the proportion of the piezoelectric properties of composite. The fillers size within the composites is also an important criterion of the composites properties. Thus, in this paper 0–3 composites, made of lead zirconate titanate (PZT) ceramic powder and polyurethane (PU) were prepared. Different sizes of ceramic grains were used: grains with average size of 10 μm, size under 80 μm and size under 100 μm. Sizes of the PZT grains was determined according to the homogenization efficiency of the composite. Piezoelectric (piezoelectric coefficient d 33 ) and dielectric properties (dielectric constant) were investigated. They have shown an interesting improvement with the increasing grains size up to 20 pC/N and 100, respectively. In order to understand the composite behavior on the microscopic scale, a simulation was carried out by mean of a finite element method (FEM) software. Finally, an estimation of the harvested voltage was modeled according to the grains size.