To see the big obstacle confronting renewable energy, look at Denmark. The small nation has some of the world’s largest wind farms. Yet because consumer demand for electricity is often lowest when the winds blow hardest, Denmark has to sell its overflow of electrons to neighboring countries for pennies—only to buy energy back when demand rises, at much higher prices. As a result, Danish consumers pay some of the highest electricity rates on the planet.
Utilities in Texas and California face a similar mismatch between supply and demand; they sometimes have to pay customers to take energy from their windmills and solar farms. On paper, wind and sun could supply the U.S. and some other countries with all the electricity they require. In practice, however, both sources are too erratic to supply more than about 20 percent of a region’s total energy capacity, according to the U.S. Department of Energy. Beyond that point, balancing supply and demand becomes too difficult. What are needed are cheap and efficient ways of storing power, to be tapped later, that is generated when winds are howling and the sun is beating down.