In this paper we investigate the effect of the contact force on the voltage generated by Contact-Mode Triboelectric Energy Harvesting Devices (CM-TEHD). The electrical energy harvested from mechanical shocks increases with the contact force. In order to investigate the role of the contact force in the triboelectric energy generation, we developed a physical model, which allows understanding the physical mechanisms of this process, while predicting the output voltage and power at given conditions. Prototypes of the CM-TEHD made of low-cost commercial silicone were fabricated using a very low cost process. The prototypes provide up to 5.5µW when subjected to repetitive impacts with a contact force of 65N.