1. A system for controlling multiple air conditioners, the system comprising: a demand control unit configured to divide the multiple air conditioners into groups, to assign a priority level to each group, to calculate an estimated power amount used by the multiple air conditioners based on an amount of power consumed by the multiple air conditioners during a predetermined time period, and to forcibly control an operation of one or more air conditioners included in a respective group based on the priority level assigned to the respective group.