In Texas, the wind tends to blow the hardest in the middle of the night. That’s also when most people are asleep and electricity prices drop, which would be a big problem for the companies that own the state’s 7,690 wind turbines if not for a 20-year-old federal subsidy that effectively pays them a flat rate for making clean energy no matter what time it is. Wind farms, whether privately owned or part of a public utility, receive a $23 tax credit for every megawatt-hour of electricity they generate. (A megawatt-hour is enough juice to power about 1,000 homes for one hour.) This credit, which was worth about $2 billion for all U.S. wind projects in 2013, has helped lower the price of electricity in parts of the country where wind power is prevalent, since wind producers can charge less and still turn a profit. In Texas, the biggest wind-producing state in the U.S., wind farms have occasionally sold electricity for less than zero—that is, they’ve paid to provide power to the grid to undercut the state’s nuclear or coal energy providers.