Agricultural biogas plants are becoming attractive in several countries, mainly due to the possibility of obtaining additional incomes by selling energy to electrical grid. However, specific conditions under which biogas plants would be economically viable are not yet well known. This paper presents results of analysis made to find optimal sizes of biogas plants in terms of electrical capacity. They were evaluated based on economic models of centralized and decentralized technological schemes and their respective mono-digestion and co-digestion systems of cattle manure, maize silage and grass silage as feedstock, for electricity generation capacities between 100 and 1000 kWe. Results show that biogas plants using mono-substrates such as cattle manure present economic viability for electrical power higher than 740 kWe. Co-digestion system presents economic viability for electrical power higher 1000 kWe. Finally, public policies related to development of these technologies, mainly in the form of subsidies, such as those existing in developed countries, could help to make the co-digestion agricultural biogas plants economically viable for the Brazilian scenario. The novelty of the paper consists in the determination of the optimum size of mono- and co-digestion patterns for Brazilian conditions showing as results the necessary level of subsidies and the paths to economic viability. (C) 2019 Elsevier Ltd. All rights reserved.