Innovative technologies and strategies to decarbonize electricity generation, transport, and heat supply sector are key factors to achieve the global climate targets set by international organizations. One of these strategies implies a significant increase of the share of renewable electricity in the energy mix. Given the intermittent behaviour of renewable energy sources (RES), a detailed assessment of future energy scenarios is required to estimate the potential surplus in electricity production. To facilitate the penetration of renewable energy sources up to significant shares, massive long-term electricity storage technologies must be considered. Among these technologies, power-to-gas (PtG) systems may foster the fossil fuels switch by providing storage of surplus renewable electricity in the form of hydrogen or synthetic natural gas. Thus, this energy carrier could be reconverted to electrical power to cover peak demand periods. In this work, a study of the prospective Spanish scenario is presented and the potential of PtG technology is assessed in terms of expected renewable surplus. We found that the annual electricity surplus for 2050 might vary between 1.4 TWh and 13.5 TWh, and the required PtG capacity would be in the range 7.0-19.5 GW, depending on the renewable production pattern and the increment of demand. (C) 2017 Elsevier Ltd. All rights reserved.