A free-piston engine generator system (FPEG) without a crank mechanism can vary the mover motion with inverter control. This study clarifies the loss-reduction effect of varying the mover motion in a dual-sided FPEG. The losses were reduced by 1 kW by setting a stroke of 160 mm, frequency of 22.5 Hz, and resonant output dispersion. Thus, we confirmed that the mover motion in a dual-sided FPEG reduces loss considering the engine loss.