In this type of market, utilities would be directed to achieve a certain percentage of energy savings. Credits would be issued when energy savings were achieved (e.g., by a utility company's customers). If a utility did not achieve its target, then it could buy the credits to offset its lack of energy savings. Once established, the credits could be sold on a secondary market.
Today's Wall Street Journal (U.S. Plays Catch-Up in Efficiency Credits, April 13, 2009), reports that, "The nascent market for efficiency gains could explode if a federal program is created, but it is unclear whether Congress will act."
Currently, the state of Connecticut has such a program and several other states are considering similar ones.