An electric power system includes: an uninterruptible power supply including an AC switch provided between a commercial power supply and an output part, a secondary battery, and an inverter provided between the AC switch and secondary battery; an important load connected to the output part; a distributed power supply connected to the output part; total load power consumption detection section detecting the power consumption of all loads including the important load; charge/discharge power detection section detecting charge/discharge power of the secondary battery; output power detection section detecting the output power of the distributed power supply; important load power consumption detection section detecting the power consumption of the important load; and a controller that inputs thereto detection values from the total load power consumption detection section, charge/discharge detection section, output power detection section, and important load power consumption detection section and outputs a control command value for controlling the secondary battery.