In this study, RAND uses a computer simulation model to assess the possible impact that a 25-percent renewable energy requirement for electricity and motor vehicle transportation could have on total national energy expenditures and on emissions of carbon dioxide by the year 2025. Currently, 6 percent of Americas energy use comes from renewable sources, including hydropower. As renewable energy supplants nonrenewable energy, demand for those fuels declines, driving down the prices of fossil fuels in the model. This generates savings in total energy cost that balance the higher cost of the renewable energies required to be used under the assumptions in the analysis. Significant reductions in carbon dioxide emissions also could be achieved with the substitution of renewable for nonrenewable energy. The authors identify potential sets of circumstances in which changes in the prices of oil, gas, coal, or renewable technologies are found to result in a reduction or increase in total energy expenditures.