In this paper we present an optimization approach for energy efficiency in redundant robotic systems. The proposed strategy leverages the positions of selected redundant joints to optimize the trajectory of the robot and minimize its energy consumption. The approach is tested on a robotic system with eight degrees of freedom, composed of a manipulator mounted on a linear axis. Extensive numerical simulations are carried out testing both a point-to-point motion and a pick-and-place task. The results show the feasibility of the proposed approach and its performance in reducing the total mechanical energy of the redundant robotic system.