On October 29, 2018, Emmett Institute faculty submitted a comment letter with and on behalf of a group of nationally renowned experts on the operations of the U.S. electric grids, in response to the recent proposal by the U.S. Environmental Protection Agency to promulgate the Affordable Clean Energy (ACE) Rule in place of the Clean Power Plan (CPP). The authors write in firm opposition to EPA’s ACE proposal. The authors argue the ACE Rule reflects an artificially constrained approach to regulating CO2 emissions from power plants, one that fights against the operations of the U.S. power system. The rule is markedly less effective than the CPP and reduces pollution only negligibly even from a no-CPP baseline. It is, at the same time, likely just as costly (or more) to industry and significantly more costly to society as a whole, forgoing billions of dollars in net benefits if the CPP is replaced by the ACE Rule.