Pigouvian subsidies are efficient, but output subsidies with uncertain or limited durations are not Pigouvian. We show that optimal time-limited policies must also subsidize investment to correct externalities generated after the output subsidy ends. Furthermore, an output subsidy’s optimal duration is characterized by the change in production when it ends. In the wind energy industry, we find that power generation decreases by 5%–10% after the end of facilities’ 10-year eligibility for the Renewable Energy Production Tax Credit. This behavioral response has implications for energy transitions and highlights how time limits could cause larger distortions in more elastic industries.