Power production from wind and solar facilities depends on the weather and not on demand for power. Therefore, it makes sense to store electricity when there is an abundance of sun and wind, in order to use it when those resources are in short supply. Looking ahead to the further expansion of renewable energies, the question arises as to when and how much power storage we will need for the Energiewende. Agora Energiewende asked a consortium of leading experts to look into this question. The focus was an examination of the costs of the power system if renewable energies were expanded to 90% in Germany.