Climate targets set by international organizations require the implementation of innovative technologies that ensure the decarbonization of energy sector. It may be partially achieved through a large penetration of Renewable Energy Sources. Massive energy storage is essential to handle excess electricity associated to RES and Power-to-Gas represents a promising option to chemically convert electricity surplus into energy carriers that may attend demands substituting fossil fuels. Aiming to avoid the influence of policies and market implications, this study approaches the decarbonization of Spanish system through the analysis of technical potential of RESrSeveral scenanos with different shares of RES are defined to cover a number of levels of energy demand. First, required wind and photovoltaic power has been estimated together with the required sizes of PtG to completely decarbonize electrical generation and industrial CHP. These scenarios may be reached by installing RES capacities below the technical potential coupled with PtG capacities between 80 and 90 GW. The stored energy amounts to 17% of total primary energy consumption. Secondly, scenarios are modified to consider denuclearization of electrical system. Required installed RES power still does not surpass the technical potential but become extremely high and the economic feasibility should be further analysed. (C) 2018 Elsevier Ltd. All rights reserved.