On June 30, one day after the Supreme Court struck down the Environmental Protection Agency’s regulation of mercury emissions from power plants, President Obama committed the United States to the goal of generating 20% of its electricity from renewable sources by 2030. This would nearly triple the amount of wind- and solar-generated electricity on the national grid.

The EPA ran afoul of the law by failing to conduct a cost-benefit analysis before it acted to reduce mercury emissions from coal-power plants. There is no objective...