This paper details the analysis of biochemical methane potential (BMP) assessment of 83 substrates, which may be deemed as: first generation substrates (food crops); second generation (grasses and wastes); and third generation (seaweed). Significant variation in the BMP of a substrate may be found depending on for example, season and method of harvest This could lead to significant discrepancy between energy production at the design stage and in operation of the facility. For example the BMP of dairy slurry varied from 175 L CH4 kg(-1) VS in autumn (cattle fed on concentrate at end of farming year) to 239 L CH4 kg(-1) VS in the summer when cattle are fed fresh grass. Grass ranged from 156 (for hay) to 433 L CH4 kg(-1) VS for first cut baled silage. Saccharina latissima (brown seaweed) generated a higher BMP 36.4 m(3) CH4 t(-1) than summer dairy slurry 16 m(3) CH4 t(-1). In terms of a national resource, the cheapest and most sustainable source of biomethane will be from wastes, but the resource is finite. Biomethane from wastes could satisfy 18.4% of transport energy in Ireland. Larger resources will require third generation substrates such as seaweed. (C) 2015 Elsevier Ltd. All rights reserved.