For a number of years, I’ve been observing demands of activists and promises of politicians that we transition our electrical grid to being supplied mainly by the intermittent renewables, wind and solar, with all large dispatchable sources (fossil fuel and nuclear) banished. Early on, I thought it was obvious that such a transition would inevitably mean that the only way to make the grid function full-time would be energy storage — on a vast scale never before contemplated or attempted. How much storage, and at what potential cost? This is actually an arithmetic problem, somewhat cumbersome but conceptually very elementary,...