Abstract: A cogeneration system including a heat engine driving an electrical generator coupled to electrical lines at a site serviced by a utility is disclosed. The system includes supervisory means for monitoring the electrical energy and/or power supplied by the generator, supplied to the site by the utility, and consumed by site electrical loads, for storing monitored electrical data, and for controlling operation of the engine and generator in response to the monitored data. The system further is provided with data representing the energy and power rate structure of the local utility, and with a real time clock, and stores monitored electrical data and controls operation of the system in accordance with whether the time corresponds to a utility peak, intermediate, or off-peak energy rate period or to a peak demand measuring period. The system further comprises means for monitoring the thermal energy transferred from the engine to a site thermal load by heat exchange means.