The amount of wind power and other time-variable non-dispatchable renewable energy sources (VRES) such as photovoltaics (PV) is rapidly increasing in the world. Several power systems in Europe are already facing a very high penetration from variable renewables which is posing concerns on the operational stability limits that are being surpassed for extreme RES generation conditions. Most transmission system operators are defining VRES limits of penetration, thus, requiring the renewable energy excess to be curtailed, exported or stored. Energy storage may play a relevant role in maximizing the long term penetration of VRES if used as a technical mean to regulate the daily, weekly and annual profiles of variable generation (VG). This paper reviews the storage technologies that are available and may be used on a power system scale and performs a cost/benefit analysis discussing their advantages and disadvantages for the integration of fast-growing renewables, such as wind power and PV.