On El Hierro Island in Spain, they set out to build an electricity system powered only by wind, with backup from a pumped-storage hydroelectric plant. The relevant statistics for El Hierro are: average electricity demand: 5 MW; nameplate capacity of the wind turbines: 11.5 MW (i.e., 2.3x “overbuild” compared to average usage); storage capacity of the pumped storage system: 70 MWh (i.e., 14 hours of average usage). The operator of the facility has provided eight years of operating data for the system. I reported on the results in my last post on October 23. In summary, the wind/storage system provided...