We study the welfare-theoretic consequences of diverging social and private time-preference rates and time-to-build for the transition to a low-carbon energy industry. We show that time-to-build, a prevalent characteristic of capital accumulation in the energy sector, amplifies the distortion induced by the split discount rates. Thus, these two characteristics create in a mutually reinforcing way less favorable circumstances for the introduction of new clean energy technologies as compared to the social optimum, even if welfare losses from emissions are internalized. We discuss resulting policy implications with particular emphasis on the energy sector.