In this paper the problem of optimal path planning and power allocation for an Unmanned Aerial Vehicle (UAV) is explored. The UAV is equipped with photovoltaic cells on top of its wings and its energy sources are solar power and rechargeable batteries. The Sun incidence angle on the photovoltaic cells, which subsequently affects energy harvesting, is determined by the attitude of the UAV and the Sun position. The desired optimal path between two given boundary points, is aimed at increasing the amount of energy storage at the final point. Meanwhile, the charging state of the battery, resulting from the power allocation, needs to be determined along with the path planning procedure. Two approaches, nonlinear programming and model reduction, are proposed and their corresponding simulation results are presented and compared.