Abstract Global warming is said to be caused by the build up of active greenhouse gases in our atmosphere and carbon dioxide — released on the combustion of fossil fuels is thought to be the most important of these. Agricultural crops, grown specifically for use as a fuel source, might play an important role in reducing our consumption of fossil fuel sources through substitution. However, in order to fully qualify as a “beneficial crop” it must be clearly shown that production entails a net benefit in terms of energy output to energy input i.e.; it has a positive energy balance. This paper considers the difficulties in assessing both carbon and energy budgeting. It describes a recently developed computer model which attempts to define the energy and carbon inputs and outputs of the leading UK energy crop, that is of short rotation coppice (SRC). Preliminary results show that SRC, when displacing fossil fuel, has a very positive energy yield with the added benefit of carbon saving in terms of offset emissions. The computer model takes account of direct and indirect energy and carbon inputs to the crop, allowing the model user to determine the sensitivity of individual cropping operations including establishment, fencing, harvesting operations, rotation and cutting cycle variations, the impact of field size, transport distance from the farm to the point of utilisation, the energy cost of storage and drying. Each management decision influences the final energy or carbon ratio of the SRC crop. Such complexity makes a definitive statement on the energy/carbon ratio for crop production difficult, however if a standard management system is adopted (as described later) the energy ratio of growing SRC for fuel may be assigned. In the case examined here a ratio of 30 : 1 (energy units produced : used) is reported. However, this ratio is sensitive to variation in crop yield and management efficiency and so, although the ratio is positive, this should not encourage a wasteful attitude to crop management systems. In terms of carbon balances, the most important consideration is the substitution value of the material in relation to fossil fuels. In this paper, figures presented refer to wood substituted for coal use and pertain to a field area of 1 hectare (ha). If field size is increased up to 3ha, then the energy and carbon ratios improve by approximately 15 %.