In recent years, data transmission by wireless communication using unmanned aerial vehicles (UAVs) has attracted attention for video transmission applications such as environmental observation. Relay UAV networks have been developed in which the observation UAV transmits data to the ground station via a relay UAV, thus allowing the observation range to be expanded beyond the range of data transmission from the observation UAV directly to the ground station. However, as the number of transmitting UAVs increases, due to interference from other UAVs, the number of UAVs in a standby state that cannot transmit data also increases. Combined with this is the existing problem in UAV data transmission caused by the limited energy storage capabilities of UAV batteries. Therefore, the energy necessary for flight must be efficiently utilized to transmit data effectively. In this paper, we consider the energy utilization efficiency of peripheral UAVs by accounting for the energy consumed by peripheral UAV flight during the time spent in a standby state due to interference from the target UAV. Then, we propose a method to maximize the utilization efficiency of the energy by controlling the transmission power and the relay UAV position in a UAV relay transmission system.