An important proposition in Economics claims that competition spurs technical efficiency, as it forces firms to raise competitiveness to survive market pressure. This study examines the effects of firm-level Lerner indexes on productivity, using a dataset on energy firms from Central European postcommunist countries during 2009-2017. The energy sector is of particular interest, as markets are still concentrated, although governments have liberalized them considerably. To contribute to the literature, I derive Lerner indexes from the production function next to involving the return on sales. Supporting the literature, the overall results highlight that market power significantly decreases productivity.