The intermittent nature of the renewable energy sources with the greater potential, wind and solar, requires dealing with temporary mismatches between demand and supply. The object of this study is to assess the Spanish energy plan from a system perspective regarding the energy storage requirements to meet electricity demand with high penetrations of renewable energy generation. We use a model that builds on existing literature and commercial software and integrates features such as demand response modelling, the correlation between reserve requirements and the technology mix, and hydrogen as an energy vector. This representation is applied to the Spanish electricity system to assess the consistency of the targets of the national energy strategy. Several scenarios of costs, demand and variation of other parameters are simulated to analyse their relative influence on the solution of minimum cost, especially assessing the sensitivity of energy storage capacity. The simulation results show that the Spanish goals for decarbonising the electricity system are based on optimistic assumptions. Also, energy storage will play a more important role than expected, and the use of hydrogen for energy storage is only needed for a 100% penetration of renewable energies.@en