Due to heavy fuel dependence, the “renewable energy system” idea is an important issue in South Korea. The government of South Korea is endeavoring to convert its energy infrastructure into renewable energy generation. As such, numerous studies have evaluated the economic feasibility of various renewable energy sources in South Korea. This study differs from those studies as it is focused on a smaller scale in terms of self-supporting nanogrid and microgrid energy. The purpose of this study is to determine which type of grid is more economical. To conduct a comparative study on the economic feasibility of nanogrids and microgrids, three substations on Jeju Island were randomly selected. We then suggested two scenarios. The first scenario is <nanogrid>, which changes each substation into a nanogrid, and the second is <microgrid>, which connects the three nanogrids to share electricity. This study identifies the optimal combination of hybrid energy resources using HOMER (hybrid optimization model for electric renewables) software to change the substations into an self-supporting energy nanogrid or microgrid. According to the net present cost and cost of energy results for each scenario through HOMER simulations, the <nanogrid> scenario is more economical than the <microgrid> scenario. However, this study also shows that microgrids can be the better option, depending on the distance between nanogrids. Finally, implications and limitations are discussed in the last section of this paper.