A power supply control method includes a step A of determining, by a power management server managing at least one facility connected to a power grid, an adjustment plan for adjusting demand and supply balance of the power grid by using at least one storage battery apparatus provided in the facility. The method includes a step B of transmitting, from the power management server to a control apparatus managed by the power management server, a control message for controlling the storage battery apparatus, based on the adjustment plan. The method includes a step C of transmitting, from the control apparatus to the power management server, reserved capacity information specifying a reserved capacity reserved as a capacity to be used for adjustment of demand and supply balance of the power grid in a storage capacity of the storage battery apparatus.