How much energy storage is needed to incorporate very large intermittent renewables?

In this paper, we present issues of electricity storage requirements based on comparative studies of various results. It was found that when we increase energy from VRE, the use of storage and its capacity increases until we reach some threshold. After that threshold, the storage use starts to decline even if we increase the size. An optimally utilized storage of about daily average demand would be sufficient to reach grid penetration of about 90% of the total demands from VRE. The understanding of the physics and economics of the future energy system is mandatory to build and operate it optimally.