We study price convergence between the two major markets for wholesale electricity in California from their deregulation in April 1998 through November 2000, nearly the end of trading in one market. We would expect profit-maximizing traders to have eliminated persistent price differences between...

Estimating market power is often complicated by the lack of reliable measures of marginal cost. Instead, policy-makers often rely on other summary statistics of the market, thought to be correlated with price cost margins---such as concentration ratios or the HHI. In many industries, these...