In May 2023, the U.S. Environmental Protection Agency (EPA) released proposed limits for carbon emissions from new and existing fossil fuel-fired power plants under Section 111 of the Clean Air Act. The proposed rules create complex incentives for generator operation, entry, and exit amid other ongoing changes to power systems. This analysis uses EPRI’s U.S. Regional Economy, Greenhouse Gas, and Energy (US-REGEN) model to understand potential impacts of EPA’s proposed rules on power sector generation and capacity mixes, emissions, and costs. The analysis suggests that EPA’s proposed rules could drive low-emitting capacity and emissions reductions beyond current trends, though the extent depends on planning uncertainties. Power sector CO2 reductions in 2040 are 65-86% below 2005 with the rules (compared with 62-76% without the rules). The proposed rules are emissions intensity standards and not technology mandates, meaning that the technologies on which standards are based may not be the ones deployed. Results across scenarios indicate that wind and solar are the largest resources in many scenarios by 2040, but natural gas capacity could a play role in regional decarbonization efforts, even as the proposed rules and other trends lower utilization. We highlight how uncertainties about impacts of the proposed rules and other drivers on costs, resource adequacy, and distributional outcomes offer opportunities for future research.