A power monitor and management system includes an automatic modulation device, a monitoring device, multiple modules and an LAN (local area network) connected to the monitoring device and the modules to enable the monitoring device to communicate with the modules. The automatic modulation device determines a dominating module in according to registration sequences of the modules and subordinating modules and to modulate power of the dominating module and the subordinating modules as well as the overall power output in accordance with number of the modules.