According to the present invention, a control apparatus for an electric generator includes a power generation cost determiner determining a power generation cost of the generator, a threshold determiner determining a threshold of power generation cost as a function of a state of charge of an electric energy storage device, a comparator comparing the power generation cost of the generator with the threshold, and a controller. When the power generation cost of the generator is lower than the threshold, the controller controls the generator to generate an increased amount of electric power, so as to both charge the electric energy storage device and feed an electrical load. Otherwise, when the power generation cost of the generator is higher than the threshold, the controller controls the generator to generate a decreased amount of electric power, so as to allow the electric energy storage device to discharge to feed the electrical load.