A system and method of conducting a computer-based, simultaneous, multiple round, descending clock auction for basic generation services (BGS) includes receiving bids by an application server host application for processing bids according to auction rules, for tranche units of BGS products, calculating next round prices, and sending round results to bidders, the subsequent round prices and a notice of the number of tranches bid during the previous round and/or the amount of excess supply offered. Subsequently, an electronic withdrawal request to remove tranches from a product and reduce a maximum number bids the bidder can make in future rounds, and/or an electronic switch request to switch a bid to another product, is received. When the number of tranches bid are equal to or less than a certain threshold for each of the products, the auction may end.