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This paper presents an optimization approach to determine the optimal sizing for a gas-fired grid-connected cogeneration system planning. An absorption chiller and a thermal storage system are included as part of the cogeneration system configuration. The optimization uses a mixed-integer nonlinear optimization technique to size the system. The search for the minimum operational cost uses a Newton-Raphson and conjugate method with tangent estimates and forward derivatives. In certain cases, the approach uses dynamic programming principle to decide on the best feasible practical solution. The electrical and thermal outputs from the turbogenerator are modeled by taking into account partial loading. The annual facility load is represented by a week of hourly load. The optimization variables used are the one week of hourly electrical energy output from the turbogenerator and the optimum solution is determined by minimizing the total operational cost. The paper evaluates three case studies of different types of load characteristic (hospital, university campus, and paper mill) to demonstrate the capabilities of the proposed model. The optimum size of the whole system will be presented. The paper will also present the financial analysis of the optimum system.