Electricity market designs that decentralize decision making for participants can lead to inefficiencies in the presence of nonconvexity or missing markets. This has been shown in the case of unit-commitment problems that can make a decentralized market equilibrium less efficient than a centrally planned solution. Less attention has been focused on systems with large amounts of hydro-electric generation. We describe the results of an empirical study of the New Zealand wholesale electricity market that attempts to quantify production efficiency losses by comparing market outcomes with a counterfactual central plan. (c) 2010 Elsevier Ltd. All rights reserved.