Currently, CCHP systems are being used over a wide area as a key alternative for producing power, heat and refrigeration. Here, a sample residential building in Mashhad city (Iran) has been selected as a case study to investigate feasibility of employing CCHP systems to meet the energy demands for various buildings sizes. An optimization algorithm is developed to find the best operation point of the Power Generation Unit (PGU) at minimum energy cost. The algorithm optimizes the operation of the CCHP systems at first step. The results of the algorithm implementation for different PGU capacities and various buildings sizes, demonstrate the performance of the operationally optimized CCHP systems at second step. The results show that CCHP system has higher performance comparing to the separate production system considering different evaluation parameters. In small buildings, primary energy saving ratio is positive for PGU capacities between 1 kW and 15 kW, with a maximum amount of 17.24%. But in large buildings, it is positive for PGU capacities between 1 kW and 30 kW, with a maximum amount of 5.1%. The PGU with capacity of 350 kW in 30-units building by 51.18% has the highest amount of energy cost saving ratio. The CCHP system with 120 kW PGU capacity in 30-units building has the least simple payback ratio by 5.08 years. Considering each parameter individually, it is showed that there are some useful mathematical relationships between optimum PGU capacity and building size. Also it is found that there is an equation for best optimum PGU capacity, considering all of evaluation criteria simultaneously. (C) 2015 Elsevier Ltd. All rights reserved.