Horizontal shifts in bid curves observed in wholesale electricity markets are consistent with Cournot competition. Quantity competition reduces the informational requirements associated with evaluating market performance because the price-cost margins of all producers then depend on the same inverse residual demand curve instead of one for each firm. We apply the model to the day-ahead market of the Nordic power exchange, Nord Pool, for the years 2011-13. We reject the null hypothesis of perfect competition in all specifications. Results suggest that the average price-cost margin across the sample period was around four percent.