Without oil, coal, and natural gas about 100 years from now, my estimate for a stable population for the U.S. is only 100 million people. The current cities will have to be abandoned without fossil energy. There will be no irrigation for food and forest production. The only regions that will be habitable will be regions where there is rainfall of about 1,000 mm per year. Each person will required about 0.5 ha of cropland for food production, 1 hectare of pasture for limited livestock, and 1 ha of forest land for biomass heat. Each house will be limited in size to about 100 sq. meters. None of the biofuels will help because they are all produced from food crops.