In the coming decades, it seems inevitable that the lion’s share of the global energy mix will come from wind and solar. But how will we keep the lights on when the wind isn’t blowing and the sun isn’t shining — especially when wind droughts can last for weeks during long, dark winter months?

Will we store huge amounts of clean energy in millions of large batteries, or simply transmit it hundreds of kilometres from regions where the wind is blowing and the sun is shining? Or will we always need gas-fired back-up power?

And there is perhaps an even more important question — how would this global back-up system be paid for when so many consumers are already complaining about the...