The deployment of large antenna arrays in Massive Multiple Input Multiple Output (M-MIMO) systems substan- tially improves mobile networks’ performance. Nevertheless, the increase in performance brought by additional antennas is often counterbalanced by an increase in Power Consumption (PC) caused by the use of supplemental hardware resources supporting additional Radio Frequency (RF) channels. In M-MIMO net- works, switching on/off RF channels and muting the associated antennas according to load conditions is known to be an efficient Energy Saving (ES) mechanism. This paper introduces a new RF channels switch on/off solution to maximize the Energy Efficiency (EE) under Quality of Service (QoS) constraints. The proposed solution is based on a MAB algorithm, which appears simpler than state of the art approaches and can be easily implemented in real systems. The algorithm leverages the quasiconcave shape of the EE metric to sequentially select the optimal antenna array configuration – i.e., the number of RF channels in both azimuth and elevation – from a predefined set of configurations. Extensive system-level simulations demonstrate that the proposed algorithm achieves a significant EE gain over a baseline solution.