The costs of biogas and electricity production from maize silage in relation to plant size are investigated in this paper. A survey of manufacturers' engineering data was conducted to derive a reliable relationship between the capacity of a combined heat and power (CHP) unit and its electrical efficiency. Then a model was developed to derive cost curves for the unit costs of biogas and electricity production and for the transport costs for maize silage and biogas slurry The least-cost plant capacity depends to a great extent on the local availability of silage maize, and ranges in the model calculations from 575 to 1150 kW(el). Finally, the paper deals with the optimum operating plant size due to the investment support available and the graduated tariff for green electricity in Austria. (C) 2007 Elsevier Ltd. All rights reserved.