How Much Electrical Energy Storage Do We Need? A Synthesis for the U.S., Europe, and Germany

Electrical energy storage (EES) is a promising flexibility source for prospective low-carbon energy systems. In the last couple of years, many studies for EES capacity planning have been produced. However, these resulted in a very broad range of power and energy capacity requirements for storage, making it difficult for policymakers to identify clear storage planning recommendations. Therefore, we studied 17 recent storage expansion studies pertinent to the U.S., Europe, and Germany. We then systemized the storage requirement per variable renewable energy (VRE) share and generation technology. Our synthesis reveals that with increasing VRE shares, the EES power capacity increases linearly; and the energy capacity, exponentially. Further, by analyzing the outliers, the EES energy requirements can be at least halved. It becomes clear that grids dominated by photovoltaic energy call for more EES, while large shares of wind rely more on transmission capacity. Taking into account the energy mix clarifies - to a large degree - the apparent conflict of the storage requirements between the existing studies. Finally, there might exist a negative bias towards storage because transmission costs are frequently optimistic (by neglecting execution delays and social opposition) and storage can cope with uncertainties, but these issues are rarely acknowledged in the planning process.