The weather-driven modeling approach of the Aarhus Renewable Energy Systems Group is applied to analyze how a combination of high-efficiency short-term and low-efficiency seasonal round-trip electricity storage can be used to enable a highly renewable pan-European power system based on wind and solar resources. In addition, the preliminary results of mapping and optimizing the interaction between storage, transmission and variable renewable energy sources in a future US power grid are presented. These results are based on data from a new global wind and solar energy atlas.