Hundreds of off-grid small power systems are located in a remote area of Indonesia, supplied by Diesel Generator (DG). The conversion plan of DG to Renewable Energy Sources (RES) generation becomes essential in increasing reliability, economic value, and environmental soundness. An example of the project is discussed through a short study on the pre-feasibility of Hybrid Renewable Energy System (HRES) utilization in Gili Ketapang island. The study considers techno-economic analysis and size optimization by using Homer Pro software. The calculation result is obtained using an assumption of fuel cost at 0.472 $/L on the studied HRES. The optimized configuration shows a better performance in the following indicators: net present cost (NPC), cost of electricity (COE), return on investment (ROI), renewable fraction (RF), and CO2 emission compared to the existing system. The study also provides the calculation that uses a nonsubsidized fuel cost assumption at 1.1 $/L. The latter result has a different optimized configuration in which ROI and environmental performances are better than the optimized configuration in the previous result.