On January 19, 2021, the U.S. Court of Appeals for the District of Columbia Circuit struck down the Affordable Clean Energy Rule (ACE), which the Environmental Protection Agency (EPA) promulgated in 2019 to replace the Obama-era Clean Power Plan (CPP). The CPP had sought to reduce greenhouse gas (GHG) emissions from existing power plants, in part, by authorizing states to increase renewable generation. As explained in a previous post, EPA had reasoned that it had the discretion to define the best system of emission reduction (BSER) at a plant under Section 111 of the Clean Air Act (Act) to include measures employed outside the facility (such as new renewable resources) that were located “beyond the fenceline.” Stayed by the Supreme Court in 2016, the CPP never went into effect. Instead, the Trump administration repealed the CPP and replaced it with ACE. In ACE, EPA reasoned that Section 111 of the Act required EPA to only find BSER to be a technology that could be applied “inside the fenceline” on the facility.