Various efforts are in progress to curb global warming and climate change. One of the solutions is to cut emission of CO2 from power plants. Decentralized power from renewable energy sources are ideal solution. This paper provides detail optimization procedure based on linear programming; for minimizing operation cost by utilizing maximum potential of REPS, and taking into consideration monthly fluctuation on generation and electricity demand. Hybrid system consisting of Hydropower, Photovoltaic and, Wind systems were taken into consideration for the optimization. Optimizations were performed for two cases, first being a remote mountainous Thingan village in Nepal and second a fairly large Ulleungdo island in South Korea. The two cases differ in demand scale, energy use, renewable energy potential, and geographic location. The optimization result show that for the HRES in Thingan, the Hydro power, Wind Power and PV power system should be 26.85 kW, 2.11 kW and 3.48 kW respectively. Similarly for Ulleungdo, optimized result for Hydro power, Wind Power and PV power was found to be 825 kW, 1291 kW and 1107 kW respectively. The optimization results indicate that the optimized hybrid system can help to completely switch from current fossil fuel dependence power system to renewable energy based power system in wide geography.