Sustainability of bioenergy products should be evaluated by means of an energy analysis that takes into account all relevant direct and indirect energy inputs. Direct energy input is viewed as the major energy consuming factor, and is quite easy to measure. Indirect energy input, however, has received relatively scant attention, so it is likely to be insufficiently analysed and possibly underestimated. This paper reviews the data available and suggests the type of research that would be needed to get a better understanding of the indirect energy input. The analysis addresses questions about the use of energy to produce and maintain agricultural machinery, the allocation of energy to different bioenergy products, and the real use and lifetime of machinery. (C) 2009 Elsevier Ltd. All rights reserved.