We develop novel statistical models of the harvested energy from renewable energy sources (such as solar and wind energy) considering harvest-store-consume (HSC) architecture. We consider three renewable energy harvesting scenarios, i.e. (i) harvesting from the solar power, (ii) harvesting from the wind power, and (iii) hybrid solar and wind power. In this context, we first derive the closed-form expressions for the probability density function (PDF) and cumulative density function (CDF) of the harvested power from the solar and wind energy sources. Based on the derived expressions, we calculate the probability of energy outage at UAVs and signal-to-noise ratio (SNR) outage at ground cellular users. We derive novel closed-form expressions for the moment generating function (MGF) of the harvested solar power and wind power. Then, we apply Gil-Pelaez inversion to evaluate the energy outage at the UAV and signal-to-noise-ratio (SNR) outage at the ground users. We formulate the SNR outage minimization problem and obtain closed-form solutions for the transmit power and flight time of the UAV. In addition, we derive novel closed-form expressions for the moments of the solar power and wind power and demonstrate their applications in computing novel performance metrics considering the stochastic nature of the amount of harvested energy as well as energy arrival time. These performance metrics include the probability of charging the UAV battery within the flight time, average UAV battery charging time, probability of energy outage at UAVs, and the probability of eventual energy outage (i.e. the probability of energy outage in a finite duration of time) at UAVs.