Passive cooling of a photovoltaic module via fins attached to the rear surface of the module is investigated. A solar module with no air cooling was used as a base model for comparison against modules cooled by the attached fins, which serve as a heat sink. The modules with fins are cooled by still air and ventilation air. A theoretical study of heat transfer through PV modules with and without fins was conducted to investigate how the calculated cell temperature and module output power are influenced by the ambient temperature and solar irradiation. The results showed a drop of module temperature and increase of electrical efficiency due to fins cooling. The electrical efficiency of the module increases significantly with the increase of fins height and number. The calculated values of cell temperature, open-circuit voltage and short-circuit current of the module with and without fins agreed reasonably with those measured experimentally over the day hours. The output power over the day hours increases at first from sunrise until noon time followed by a decrease until sunset. On the contrary, the electrical efficiency decreases from sunrise until noon time followed by an increase until sunset in agreement with previous findings in the literature.