The article presents the conceptual design of Hybrid Energy Source System (HESS) for Long Endurance Electric UAV. During the design, the MBD method was used to analyze energy consumption during various UAV operational states as well as the entire mission for different HESS concepts. Such analysis at an early stage of the concept allows you to choose the optimal HESS set for a given UAV and expected mission parameters. The design method is based on a simulation computer model of subsystems and flight phenomena affecting the energy consumption during the flight. The modular construction of the simulation model allows the use of various details of simulation models for different stages of project development. The use of the HESS solution as well as the method of its design makes it possible to overcome the basic inconvenience of using electric drives in all types of vehicles, ie reconciling sufficient temporary operational parameters of the drive and long-term operation. In addition to the description of the method itself and the details of the simulation model, the article also presents a case study for the popular UAV class.