The U.S. Environmental Protection Agency is currently drafting existing source performance standards under Section 111(d) of the Clean Air Act. However, there is considerable uncertainty about their form, timing, and interactions with other possible climate policies and incentives. This analysis uses EPRI’s U.S. Economy, Greenhouse Gas, and Energy (US-REGEN) model to assess potential implications of performance standards for the power sector generation mix, emissions, and costs. The results suggest that performance standard design decisions—especially their flexibility and trading provisions, form, and stringency—materially alter electric sector outcomes. Existing source standards can accelerate reductions of CO2 emissions and criteria pollutants. However, extended tax credits lower CO2 considerably more than the regulations studied here and also can lower prices for customers by shifting costs from electricity bills to the federal tax base.