A calibration system for a generator set controller is disclosed. The calibration system may have a source of electric power, a controller in communication with the source and configured to monitor a parameter of the electric power and produce at least one output corresponding to the monitored parameter, and a power meter configured to measure the parameter of the electric power. The calibration system may also have a processor in communication with the source of electric power, the controller, and the power meter. The processor may be configured to direct electric power having a first test voltage from the source to the controller, and directly relate a value of the at least one output corresponding to the first test voltage and a value of the measured parameter to an offset error.