Abstract: A system for monitoring power usage includes a power distribution unit (PDU) and a monitoring device. The PDU includes a detecting module and a storing module, the monitoring device includes an amplifying module and a processing module. The detecting module obtains signal as to voltage and current relating to a power supply, and the storing module stores data as to calibration. The amplifying module obtains the signal as to voltage and current, and amplifies the same. The processing module obtains the amplified signal and processes the amplified to obtain data as to electrical flow. The processing module can obtain calibration data from the storing module, and apply corrections according to the calibration data.