Short- and long-run marginal cost pricing: On their alleged equivalence

Abstract

The equivalence between short-run marginal cost (SRMC) and long-run marginal cost (LRMC) in a fully adjusted equilibrium has been proved over and over again. In the literature dealing with public utility pricing, this basic result has been taken to imply that it is optimal to set prices at LRMC. Important contributions to this literature are reviewed in this paper. The equivalence, however, is valid only under the very restrictive assumption that the capacity can be varied continuously. This means that indivisibilities, irreversibilities and durability of investments are ignored. Where such phenomena exist, as in electricity production and distribution, pricing according to LRMC is neither theoretically valid nor applicable. It is not surprising that it has been difficult for public utilities to define the LRMC concept operationally; average cost concepts are used as ‘approximations’. Under these circumstances we find it advisable to dispense with the LRMC concept altogether and rely on pricing based on SRMC.