Many regions in Indonesia are still lack of electricity due to geographical condition in the form of archipelago, including Tanjung Balai Karimun District, in Kepulauan Riau Province, Indonesia. Therefore, using local renewable resources to fulfill load demand is better than relying on utility grid. The potential of using biomass waste, for instance oil palm and coconut residues, as renewable energy resources should be an optional decision to make improvement, not only for the reliability of existing power system itself but also to meet the load demand generally available in every region of Indonesia. In 2016, Karimun Regency in Riau Islands Province as a Special Economic Zone (SEZ) is growing rapidly in economic and facing electrical energy crisis simultaneously. Addressing this problem, the government has taken an action by making amendment on the policy that consciously planned to increase the capacity of power plants, in particular Oil and Gas Power Plants (OGPP) in the period of 2018-2024. The objective of this study is to evaluate the reliability of Tanjung Balai Karimun power system based on its Loss of Load Probability (LOLP) index designed during the planning period of power plant addition. Then, the value of new LOLP index was recalculated after integrating new system of Biomass Power Plant (BPP). Based on LOLP calculation using recursive method, LOLP index in 2018 decreased from 3.63 days/year to 2.05 days/year, reflecting quality improvement of the power plant reliability. Integrating BPP into the existing power plant has powered up the capacity of 2.8 MW according to the availability of oil palm and coconut biomass waste.