This paper considers power assist control for periodic motions. In most power assist applications, additional force is applied in proportion to the instantaneous value of force generated by human. However, human persistent tasks are sometimes periodic and it can be shown that the proportional assist is not optimal for such motions in the sense of energy efficiency. Regarding this aspect, we proposed several power assist methods for periodic motions based on repetitive control in our previous works. In this paper, we compare the energy efficiency of each method through experiments with electric power-assisted bicycle.