This paper analyzes the impacts of imposing a Federal 20 percent non-hydropower renewable generation portfolio standard (RPS) on US energy markets by 2020. The US currently has no RPS requirement although some state RPS regulations have been adopted but not uniformly enforced (see http://www.eia.doe.gov/oiaf/analysispaper/rps/index.html for a recent summary on RPSs in the US). The renewable portfolio standard (RPS) requires that 20 percent of the power sold must come from qualifying renewable facilities. The analysis of the 20 percent RPS was developed by using the December 2001 version of the National Energy Modeling System (NEMS) of the Energy Information Administration (EIA) and the assumptions and results of the Annual Energy Outlook 2002 (AEO2002) reference case. 2 A policy that requires a 20 percent non-hydro-electric RPS by 2020 appears to be effective in promoting the adoption of renewable generation technologies while also reducing emissions of nitrogen oxides by 6 percent, mercury by 4 percent and carbon dioxide by about 16.5 percent relative to the reference case in 2020. Electricity prices are expected to rise about 3 percent while the cost to the electric power industry could rise between 35 and 60 billion dollars (in year 2000 dollars in net present value terms). (c) 2006 Elsevier Ltd. All rights reserved.