There are numerous ways to assess and compare biofuels. Gross energy per hectare reflects the quantity of product produced per unit of land. Net energy per hectare reflects the parasitic demand associated with the product per hectare. Gross and net energy per hectare are far superior for grass biomethane than rape seed biodiesel. For a biofuel made from residues the descriptor MJ GJ(fuel)(-1) (MJ of biofuel produced per GJ of fossil fuel displaced) is more instructive; this reflects the relative efficiency of the biofuel. Of issue in the assessment is how to deal with co-products, by-products and residues. The allocation methodology allows for a variety of answers to be generated. UCO biodiesel has a good energy balance for any allocation approach; tallow biodiesel has a poor net energy unless credit is given for the co-production of meat and bone meal as a substitute fuel. To be deemed sustainable by the EU Renewable Energy Directive a value of 60% GHG savings is required for facilities built post 2017. A further crucial consideration is: how much fuel can be produced? This study shows that indigenous biodiesel produced in Ireland and grass biomethane may be deemed sustainable but only grass biomethane may produce a significant quantity, potentially satisfying the 10% renewable energy in transport target for 2020 as opposed to only 1.23% in total from all indigenous biodiesel systems. (C) 2010 Elsevier Ltd. All rights reserved.