Grid-scale electricity storage, in the long run, could be a boon to intermittent energy sources like wind and solar. However, in the short run, cheap energy storage could help coal plants operate even more efficiently, allowing them to store energy at night and sell more during peak demand. If grid storage breakthroughs happen without cheaper clean power, GHG emissions could rise slightly. But when combined with clean power innovation in the All Tech Breakthrough scenario, storage increases renewables use, reducing CO2. By 2050, storage helps intermittent wind and solar reach 48% of total US electrical generation.