This study proposes an optimal installed capacity of Distributed Energy Resources (DERs) for a small remote island (Aguni-Island) is belongs to Okinawa Prefecture in Japan. Photovoltaic (PV), Wind Generator (WG), Battery Energy Storage System (BESS), and Fuel Cell (FC) are considered to be installed for optimal sizing. The simulation conducted in this study has been simplified to account seasonal load and weather variations. This simulations aims to minimize the fuel and total cost of the system as it resulted in a total cost, which is lower than it was before DERs to be implemented. In addition, carbon dioxide emissions from diesel generators have been reduced.