A server for a charge-discharge system, the server comprising a controller configured to communicate with a plurality of registered vehicles each equipped with a chargeable-dischargeable battery, the controller being configured to: acquire information on a scheduled start time of a next travel of each of the registered vehicles, acquire electric power information, the electric power information being one of electric power system information that shows a demand-and-supply relationship of electric power in an external electric power system and an electric power adjustment request that is issued based on the relationship, determine, based on a scheduled travel start time and the electric power information, a target vehicle of which the battery is to be charged or discharged from among the registered vehicles that are at least parked, send to the target vehicle one of a charge command that orders the battery of the target vehicle to be charged and a discharge command that orders the battery of the target vehicle to be discharged to the external electric power system; calculate, based on a current time and a charge start time that is determined according to the scheduled travel start time, an available time of each of the registered vehicles that is a time left from the current time until the charge start time, determine the target vehicle based on the available time and the electric power information; and determine, as the target vehicle, the registered vehicle of which the available time is not shorter than a predetermined time; characterised in that the controller is configured to determine, as the target vehicles, a predetermined number of the registered vehicles in ascending order of the available time. the server comprising a controller configured to communicate with a plurality of registered vehicles each equipped with a chargeable-dischargeable battery, the controller being configured to: acquire information on a scheduled start time of a next travel of each of the registered vehicles, acquire electric power information, the electric power information being one of electric power system information that shows a demand-and-supply relationship of electric power in an external electric power system and an electric power adjustment request that is issued based on the relationship, determine, based on a scheduled travel start time and the electric power information, a target vehicle of which the battery is to be charged or discharged from among the registered vehicles that are at least parked, send to the target vehicle one of a charge command that orders the battery of the target vehicle to be charged and a discharge command that orders the battery of the target vehicle to be discharged to the external electric power system; calculate, based on a current time and a charge start time that is determined according to the scheduled travel start time, an available time of each of the registered vehicles that is a time left from the current time until the charge start time, determine the target vehicle based on the available time and the electric power information; and determine, as the target vehicle, the registered vehicle of which the available time is not shorter than a predetermined time; characterised in that the controller is configured to determine, as the target vehicles, a predetermined number of the registered vehicles in ascending order of the available time. acquire information on a scheduled start time of a next travel of each of the registered vehicles, acquire electric power information, the electric power information being one of electric power system information that shows a demand-and-supply relationship of electric power in an external electric power system and an electric power adjustment request that is issued based on the relationship, determine, based on a scheduled travel start time and the electric power information, a target vehicle of which the battery is to be charged or discharged from among the registered vehicles that are at least parked, send to the target vehicle one of a charge command that orders the battery of the target vehicle to be charged and a discharge command that orders the battery of the target vehicle to be discharged to the external electric power system; calculate, based on a current time and a charge start time that is determined according to the scheduled travel start time, an available time of each of the registered vehicles that is a time left from the current time until the charge start time, determine the target vehicle based on the available time and the electric power information; and determine, as the target vehicle, the registered vehicle of which the available time is not shorter than a predetermined time; characterised in that the controller is configured to determine, as the target vehicles, a predetermined number of the registered vehicles in ascending order of the available time.