An economic evaluation model was developed in the Laboratory of Agricultural Structures (LAS) of the Agricultural University of Athens-the Modified Basic Economic Evaluation Model (MBEEM). This model is an improved version of the original Basic Economic Evaluation Model, available in LAS, and it is used to assess the cost-effectiveness of biogas production systems. Because of the parameters involved, a computer model was developed to facilitate the application of the MBEEM. The model was used in this work to determine the cost-effective size of a local centralized biogas production system fed with pig wastes.