The compared energy analysis of various farming systems makes it possible to carry out an approach of their sustainability through their faculty to produce energy in the form of livestock or vegetable products, while limiting the resort to non renewable energies. This study was carried out from the results of 38 farms in meat sheep and dairy cow production including 13 in organic farming. It focuses on livestock production, being acquired that the presence of cash crops raises considerably the energy efficiency. The main energy expenditure are the bought food, fertilisers and mechanization. The average energy efficiency of the dairy cow farms is 0.59 and there is no difference between organic and conventional farming. In sheep production, the average is lower (0.41) but the organic farms have a higher level (0.47) thanks to a better food self sufficiency and no use of chemical fertilisers.