Many developed countries are moving towards a low carbon economy and are therefore demanding higher levels of renewable energy sources. These energy sources include wind, solar, biomass, etc. to supply the energy demand. Nevertheless, there are still some aspects that warrant further technical and economical feasibility studies for those renewable energy sources to be considered sustainable alternatives. The random nature of renewable energy sources, mainly solar and wind is the major limiting factor in achieving significant penetration in any electric system. This limiting factor has different consequences depending on the ratio between the amount of renewable generation and the demand level. This has been studied by many authors from different perspectives and in many cases the key element was energy storage. For example, energy storage can be used to reduce the production fluctuations of large scale wind farms, to move a certain production amount to better remunerated periods, to reduce prediction errors to minimize penalties, to increase the power predictability, to participate in secondary power markets and to achieve fully controllable energy production through any renewable primary source. In such way, any renewable generator would offer guaranteed production and may participate in electric markets on equal terms with non-renewable generators. Most analyses of isolated large electric systems with renewable supply and storage are performed based on energy balance results over several years (Bremen et al., 2009) (Alonso et al., 2009, 2010). This methodology has been extended in order to include real data measurements from various renewable technologies and also storages with different dynamic and rates. In particular, this method allows the resolution of multiple scenarios of renewable penetration levels, profiles, technologies, etc in order to obtain the minimum storage service that will reduce the conventional production or even satisfy a total supply of the demand through only renewable producers. This methodology has been firstly used to analyse a suitable large system: the Spanish electric system. Although Spain is electrically connected with other countries, the low rates in exchanged energy allow the simplification to consider it as an isolated system. As it will be shown, Spain offers excellent opportunities to produce large amounts of renewable production. However, the