Industry is more and more urged towards energy efficiency by the increasing societal and environmental concern about energy. This is also true for more recent production system types, like Reconfigurable Manufacturing Systems (RMS), which are gaining momentum due to the advent of Industry 4.0 and the growing uncertainty of markets. In this work, we consider energy efficiency in the design stage of an RMS. The tasks of a production process must be assigned to the machines of a Parallel-Serial manufacturing line, and scheduled, so that the related power consumption peak is minimized. An Integer Linear Programming formulation is proposed and tested on a set of benchmark instances. Numerical results are discussed.