The advantages of economies of scale in marine logistics and transportation can be obtained by building bigger ships. As ship sizes increase, so do their propulsion systems, requiring that stable and high-efficiency power must continuously be supplied. In general, ships’ operations require power lower than the installed generator capacity. However, when the generator is operated at a low load, its efficiency decreases. In this study, based on actual operation data, the load requirements for each operation mode were analyzed, and a diesel-generator-based power system was designed. We present a generator capacity optimization calculation method through generator capacity. The proposed strategy maximizes the space utilization and efficiency of the ship while minimizing the generator’s power consumption. The generator’s fuel consumption, operating time, and efficiency were compared and analyzed to verify the proposed strategy’s efficacy. In conclusion, the proposed strategy demonstrated the effect of reducing fuel consumption by 2.2%, increasing generator efficiency by 8.4%, and reducing costs by 5.14% compared to the existing onboard generator capacity for the same vessel.