This paper investigates three important energy and climate policy issues: (1) the availability of biomass for electricity generation (i.e., supply), (2) climate policy effects on this supply, and (3) the net greenhouse gas reduction when biomass is used for electricity generation. Using a detailed model of U.S. agriculture and forestry markets and land-use, that includes a broad and diverse set of biomass feedstocks, we evaluate competing potential sub-national and feedstock specific supplies of biomass for U.S. electricity generation. Our preliminary results suggest significant supply, with residues dominating at lower delivered energy prices, and dedicated crops significant at higher prices. Sub-national variation is dramatic and will affect generation siting and sustainability. We find displacement of food crops, but net forest land and cropland expansion. We also find that GHG policies could substantially increase the delivered cost of biomass; however, the implications for individual regions and feedstocks is non-uniform, with some supplies falling to zero and others increasing. Finally, we find that bioelectricity is not carbon neutral, but can be emissions reducing relative to coal generation, yield greater direct GHG benefits than biofuels, and even result in domestic indirect emissions reductions with incentives for forest based feedstocks.