A communication part of a purchased power management device transmits, in response to an instruction from a user, rate information indicating a rate of power that the user wants to purchase to a sales management device. A charging part of the purchased power management device charges the battery at a date and time designated by the sales management device. A power output part of the purchased power management device outputs power stored in the battery during a time period of high electric power demand. A sale destination decision part of the sales management device decides a sale destination of the power generated during a time period of low electric power demand on a basis of the rate indicated by the rate information. A sales date and time information transmission part of the sales management device transmits sales date and time information designating a date and time when power is sold to the sale destination decided by the sale destination decision part.