The Clean Air Act has assumed the central role in U.S. climate policy, directing the Environmental Protection Agency to develop regulations governing the emissions of greenhouse gases from existing coal-fired power plants. The cost and environmental effectiveness of policy options depend on abatement costs, the magnitude of emissions reduction opportunities, and the sensitivity of plant utilization. This paper examines the operation of electricity-generating units over 25 years to estimate the marginal costs and potential magnitude of emissions reductions that could result from improvements in their operating efficiency. We find that a 10 percent increase in coal prices causes a 0.3 to 0.9 percent heat rate reduction, broadly consistent with engineering assessments of abatement costs and opportunities. We also find that coal prices have a significant effect on utilization, but that will vary depending on the policy design. The results are used to compare cost-effectiveness of alternative policies.