To provide a power control method, a power control system, a controller, and a computer program which reduce energy cost, and suppress an inverse load flow to a commercial power source, when a plurality of power systems are liked in order to supply, to power load, low-cost power generated by a private generator and expensive commercial power purchased from a power company, and operation is performed by constant received power control, which makes the received electrical energy approach the target value.

A received electric power value which represents electrical energy received every unit time is accepted (S102), and the accepted received electric power value is compared with a minimum received electric power value which is a reference value for lower limit of a received electric energy per unit time (S103). If the received electric power value is determined as being to or lower than the minimum received electric power value (S104: Y), a target value for the received electrical energy is reset to a higher value (S105). If the received electric power value is determined as being larger than the minimum received electric power value (S104: N), and in addition, if a counted time is determined as having reached a specified time (S108: Y), the target value for the received electrical energy is reset to a lower value (S109).