Energy efficiency subsidies with price-quality discrimination

We compare various designs of energy efficiency subsidies in a market subject to both energy-use externalities and price-quality discrimination by a monopolist. We find that differentiated subsidies can establish the social optimum. Unlike per-quality regimes, ad valorem regimes generate downstream interferences: Subsidising of the high-end good leads the monopolist to reduce the quality of the low-end good. For this reason, ad valorem differentiated rates should always decrease with energy efficiency, a result seemingly at odds with actual practice. In contrast, with per-quality differentiated subsidies, the rates can increase if the externality is large enough relative to the market share of "low" type consumers. Contrary to differentiated subsidies, what we shall call single-instrument subsidies only achieve second-best outcomes. A uniform ad valorem subsidy should have a rate higher than that needed to specifically internalise energy-use externalities. Lastly, if, as is often observed in practice, only the high-end good is to be incentivised, a per-quality regime should be preferred to an ad valorem one. An ad valorem tax on the high-end good may even be preferred to an ad valorem subsidy if the externality is small enough and low-end consumers dominate the market. © 2015 Elsevier B.V.