The aim of this article is to determine the size of agricultural biogas plants which may arise in farms taking into account the current experience and trends of the Law on renewable energy sources. It was found that only the largest farms (more than 200 heads of cattle and 500 pigs each) can provide sufficient substrates for the operation of a biogas plant with a capacity of 1 MW el. For this reason it seems preferable founded in the draft of RES Law on the promotion of RES installations consumer up to 40 kWel .