Piezoelectric thin films are widely used in MEMS and NEMS actuators and resonators, but also in mechanical sensors and energy harvesters for IoT applications and Wireless Sensors Networks. Nanotechnology involving piezoelectric materials is a key research direction, with benefits expected from nanostructuring and the replacement of toxic materials. Piezoelectric nanocomposites based on semiconducting nanowires (NWs) are an alternative to thin films with nanostructuration benefits, such as low temperature fabrication and higher flexibility than thin films. In addition, they exhibit larger piezoelectric coefficient than their thin films counterparts. In this work we study the piezoelectric performance of vertically grown ZnO NWs based on Finite Element simulations in the PFM (Piezoresponse Force Microscopy) configuration. In this AFM (Atomic Force Microscope) mode, the AFM tip is placed in contact with the top surface of the NW while applying a voltage, thus inducing a deformation of the structure by the reverse piezoelectric effect. Different parameters are assessed: the effect of the surrounding air, the NW size and geometry and the effect of the semiconducting properties, in particular the doping level and surface traps density. The results are compared to previous theoretical approaches and experimental findings.