An electric storage system to which a first electric storage device and a second electric storage device connected in parallel can be mounted decides a rated voltage of the first electric storage device in an electric storage system having at least one of (A) a discharge standby stage and (B) a charge standby stage. It decides a large and small relationship between the rated voltage of the electric storage device for which at least one of (i) an accumulated time, (ii) an accumulated power amount, and (iii) an accumulated number is decided to be made higher among the first electric storage device and the second electric storage device, and the rated voltage of the other electric storage device. It decides the rated voltage of the first electric storage device such that the large and small relationship decided in the large and small relationship deciding step is achieved.