Motivated by a desire to reduce carbon emissions, and in the absence of federal action to do so, 29 states (and the District of Columbia and Puerto Rico) have required utility companies to deliver specified minimum amounts of electricity from "renewable" sources, including wind and solar power. But this patchwork of state rules—which now affects the electricity bills of about two-thirds of the U.S. population as well as countless businesses and industrial users—has sprung up in recent years without the benefit of the states fully calculating their costs.