The unmanned aerial vehicle (UAV) cellular communications is a promising technique to boost coverage and realize intelligent applications. The communication power consumption is a major challenge faced by the UAV cellular networks. In this work, we consider a single UAV user equipment (UE) connected with a terrestrial base station (BS), and focus on the uplink (UL) performance. Through theoretical analysis, we find an optimal power control factor in terms of the communication energy efficiency, when the distance between the UAV and the BS is small.