When renewable energy sources such as solar and wind farms generate more electricity than consumers need, storing the excess doesn’t always make sense, say researchers from Stanford University. Large, grid-scale batteries capable of storing the extra electricity are resource-intensive and costly to manufacture and maintain — sometimes more so than the energy they’re used to store. “You wouldn’t spend a $100 on a safe to store a $10 watch,” said Michael Dale, who co-authored the study in the journal Energy & Environmental Science. “Likewise, it’s not sensible to build energetically expensive batteries for an energetically cheap resource like wind.” Economically, it makes more sense to shut down wind energy production when consumer demand is low than it does to maintain battery systems to store excess wind energy, the study said. But battery storage does make sense for photovoltaic systems, the researchers say, because solar panels and solar farms require more energy to build and maintain.