This paper examines more than a century of U.S. power plant additions and retirements in conjunction with several decades of utility capital investment data. While policy analyses often invoke assumptions of power plant book life, relatively little analysis has focused on the physical life of power-generating assets. The average age of the U.S. generator fleet has increased significantly over time despite continued investment, in part because more recent investment has tended to focus on shorter-lived assets. This may be due in part to risk-averse power sector investors and lenders responding rationally to regulatory uncertainty in a deregulated market environment. Power plant retirement trends suggest that the pace of retirements will increase significantly in the decade after 2030 for most reasonable estimates of physical life. These capital investment trends have important consequences for carbon policy and highlight the importance of including consideration of the longer term particularly when evaluating more significant decarbonization policies.