This article presents the results of work of power supply system of an unmanned aerial vehicle (UAV) powered by solar cells. The UAV power supply system consists of solar cells, a charge controller, battery cells and a BMS (Battery Management System). During the designing process various options for energy acquisition and recovery was considered, in particular ATG (Advanced Thermoelectric Generator). The MBD (Model-Based Design) methodology was used to develop the UAV power supply system. The system was developed in simulation model and next it was studied to find the space of possible solutions using this model. Solar cells are the most efficient if the sun rays fall on them perpendicular. During the simulation various angles of inclination of solar cells in relation to sun rays were studied. These values depend on latitude, azimuth, season (length of day), weatheri.e. if there are any clouds and even air pollution. The power supply system had to be constructed in such a way to ensure during the day excess to energy enabling the operation of the engines, peripheral devices (sensors, measuring devices, GPS module) as well as charging the batteries to maximum capacity. The next step was related to the proper selection of battery cells to ensure the operation of the devices and flight at night. The whole research was additionally extended by minimizing the mass of power supply elements while increasing the ability to achieve energy autonomy. The developed system allows to increase the UAV flight duration, and with appropriate construction, geographical location and favorable weather conditions it is able to provide full energy autonomy of the UAV. The UAV powered by solar cells enables for example monitoring of pollution, boundaries, power lines, crops and measuring selected physical quantities over any area e.g. smog.