1. A power generation leveling system that utilizes power generated during a time period of low electric power demand, comprising: a sales management device that decides a sale destination of the power generated during a time period of low electric power demand; a purchased power management device including a battery storing purchased power; an electric energy measurement part that measures electric energy stored in the battery by the charging part of the purchased power management device; and an information transmission part that transmits purchased electric energy information indicating a result of the measurement by the electric energy measurement part to the sales management device, wherein the purchased power management device includes a communication part that, in response to an instruction from a user, transmits rate information indicating a rate of power that the user wants to purchase to the sales management device; a charging part that charges the battery at a date and time designated by the sales management device; and a power output part that outputs power stored in the battery during a time period of high electric power demand, wherein the sales management device includes a sale destination decision part that 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 that transmits sales date and time information designating a date and time when power is sold to the sale destination decided b the sale destination decision part; and a rate calculation part that calculates an amount billed to an installer of the purchased power management device on a basis of electric energy indicated by the purchased electric energy information transmitted by the information transmission part and the rate indicated by the rate information transmitted by the communication part of the purchased power management device.