Climate Change Economics, Ahead of Print. Renewable electricity subsidies have been popular policy instruments to combat climate change because of their ability to offset emissions. This paper studies the long-run welfare benefits of optimizing the design of the existing renewable energy subsidy (the status quo) in the presence of heterogeneity in the offset emissions. In particular, I measure the welfare gain from differentiating renewable subsidies across location and time to reflect the environmental benefits from emissions offset in the context of wind energy in the Texas electricity market. I find that the welfare gain from differentiation is small compared to the gain already achieved under the status quo subsidy. In contrast, the optimal emissions tax yields much larger welfare gain because it engages in other cost-effective emissions abatement channels that renewable energy subsidies do not: namely, demand conservation and cross-plant fuel substitution.