An efficient electricity double-sided auction mechanism should control market power and enhance the social welfare of the electricity market. Based on this goal, the paper designs a new double-sided auction mechanism. In the new mechanism, the social welfare contribution of each participant plays a pivotal role, because this contribution is the critical factor in market clearing, payment settling, and transaction matching rules. In particular, each winner of the auction can gain transfer payments according to his contribution to social welfare in the electricity market, and this gives the mechanism the ability to control the market power of some participants. At the same time, this mechanism ensures that the market organizer balances his budget. We then conduct a theoretical and empirical analysis based on the Spanish electricity market. Both of the results show that compared to the uniform-pricing mechanism, the new mechanism can reduce market power of participants and enhance the social welfare of the electricity market. (C) 2009 Elsevier Ltd. All rights reserved.