To authorize Federal agencies and legislative branch offices to purchase greenhouse gas offsets and renewable energy credits, and for other purposes.

Authorizes federal agencies and legislative branch offices to purchase qualified tradeable: (1) greenhouse gas (GHG) offsets; and (2) renewable energy credits.

Requires the Secretary of Energy to promulgate rules regarding approved open market transactions involving such offsets and credits. Requires such rules to accommodate forward purchasing and crediting of offsets and credits on an estimated basis from small scale offset and renewable energy generators and other open market transactions that help enable short-term purchases of offsets and credits to contribute to the implementation of such generators.

Defines "greenhouse gas offsets" to mean a reduction in GHG emissions that results from actions that would not have occurred in the absence of the opportunity to sell an offset for the resulting reduction.