To utilize an electrical power for heating water efficiently.

A control device of an electric water heater using an electrical power to boil up includes: a switching control means that performs switching control so that one of the power of a system 302 and the power of a storage battery 301 is supplied to the electric water heater as a driving power; and a means to acquire a system ratio that indicates the ratio of an amount of hot water that should be boiled when the system power is supplied to the electric water heater based on an ambient temperature among a prescribed amount of hot water that should be boiled by the electric water heater and a battery ratio that indicates the ratio of an amount of hot water that should be boiled when the power of the storage battery is supplied to the electric water heater.