Abstract We assess the energy contents of agricultural and forestry residues in Spain, and the potential for the generation of electricity from them. The methodology employed is a hierarchical, GIS-based one, and leads through the physical, geographical and technical potential to the economic analysis. The results from the latter are crafted in the form of generation-cost curves, which provide a good indication of how the cost of the energy increases as the generation from these residues does. Geo-referenced data allow for the consideration of the opposing influences on the specific cost of the plant size and the transport costs, which are both incorporated in the model by means of a plant supply area. A representative cost is defined and used to compare costs among biomass sources and combustion technologies. The combined technical potential of agriculture and forestry residues is 118 PJ y−1 (equivalent to 11.25% of the net electric energy generated in Spain in 2008). The economic potential (defined as the potential with a cost smaller than the representative one) is 46.3 PJ y−1 (or 4.43% of net electric energy generated in Spain in 2008).