The implementation of micro-cogeneration plants in residential buildings requires a technical and economic viability study. This analysis depends greatly on the regulatory framework controlling this kind of installations, which is characterized by its variability and great uncertainty. Viability is also closely related to the sizing of devices and their integration within the plant, as well as to its global operation. Although different methods are used for sizing micro-cogeneration installations, there is no methodology to determine the optimal capacity of the thermal energy storage and the auxiliary generation system in the design phase. Since the optimal strategy of the whole plant is not taken into account in this project phase, the installation is usually oversized, decreasing the efficiency of the plant and increasing the overall cost. The aim of this paper is to analyze the viability study of micro-cogeneration systems with integrated thermal energy storage and determine the influence of this on the final results. Furthermore, a mathematical linear programming-based model is proposed, where the optimal behavior of the different devices is predicted in the design phase in order to determine the optimal sizing of both the tank and the auxiliary boiler. The developed model can be a useful tool in viability analysis and can easily be reproduced by engineers and researchers. In conclusion, the optimal integration and sizing of the thermal energy storage considerably improve the thermodynamic, economic and environmental results This work was supported by the Spanish Ministry of Science, Innovation and Universities and the European Regional Development Fund through the MONITHERM project ‘Investigation of monitoring techniques of occupied buildings for their thermal characterization and methodology to identify their key performance indicators’, project reference: RTI2018-096296-B-C22 (MCIU/AEI/FEDER, UE)