Abstract The aim of this study is first to make a critical evaluation of the present situation of the power system in Spain making use of the latest available data. The other main objective is to make an analysis of the optimal mix for the intensive deployment of variable renewable energies (VRES). The VRES considered are wind, solar photovoltaics (PV) and concentrating solar power (CSP), which by 2030 should provide all together about 45–50% of the total electricity demand in order to comply with the European Union (EU) Directives and the Paris 2016 Climate Agreements. As we will show, these levels of VRES would create important problems of surplus and needed backup energies, especially in the lack of sufficient electricity storage facilities. Therefore, it will be needed during the transition period the parallel use of fossil conventional backup plants, with their corresponding associated emissions. By applying linear programming optimization techniques we try in this paper to make optimal the large deployment of VRES. By “optimal” we mean that the shares of VRES would simultaneously minimize both the backup and the surplus power components for the whole set of hours (8760) in the year. For the most viable Scenario to be implemented around 2030, we have also studied the statistical distribution of slacks (surpluses and backups) so that the demand could always be covered, providing a valuable data for the provision of additional storage. In summary this scenario assumes a large reduction of coal power plants, the continuation of present nuclear power plants and the preservation of combined cycle power plants which will be used for backup due to their high flexibility and ramping speed.