The present invention relates to a power system utilizing an engine generator or public power with AC source, wherein the power system particularly has the characteristics that the maximum output current thereof is limited by electromagnetic effects, and/or constant current or nearly constant current output thereof is set to be lower than the maximum output current, for powering a load, and charging a battery, or jointly powering a load with a battery; when an engine generator set is utilized to be power supply, during operation, the engine operates with the best brake specific fuel consumption and/or the range of revolutions and torque for better energy saving.