In the regions where electricity supply capability is insufficient or power grid is unstable,ports need to be powered by the port’s self-powered station. According to the scale of port’s electricity consumption, we generally use diesel generators as the self-contained power. It is critical for the normal operation of the port in these regions to determine the generator capacity accurately. Based on the analysis of electrical equipments in the port, and combining with the definitions of the power ratings in relevant standards, we propose a method to determine the capacity of the diesel generator in the port.