We have proposed a method for selecting optimal energy storage systems (ESSs) for off-grid wind-hybrid power systems in South Korean islands, in terms of economic factors and performance. These islands are classified according to their sizes. The technical specifications of various batteries such as flow batteries, lead-acid batteries, and Li-ion batteries in the optimized wind-hybrid systems of target islands were analyzed. The flow battery was found to be the best ESS for most of the test islands under the current circumstances. We have also evaluated several scenarios with varied prices and technical specifications of the batteries to determine the factors that affect the selection of the optimum ESS. As a result, we found that the nominal energy of the batteries is the most important factor that assures economic competitiveness of the system.