Via Campesina supports peasant and small farmer agriculture both in the South and in the North. Its basic doctrine is that of ‘food sovereignty'. It is a movement that defends an ‘ecological neo-Narodnism'. Among the analytical tools used by this international peasant movement is the comparison between the energy efficiency of traditional small farm agriculture and modern industrial agriculture. This article briefly recalls the history of agricultural energetics, and then looks at the use of the concept of EROI (energy return on energy input) by Via Campesina when it claims that ‘industrial agriculture is no longer a producer of energy but a consumer of energy', and that ‘peasant agriculture cools down the Earth'. The absence in Marxism of a tradition of analysis of energy flows is also reviewed here, since it is of interest in order to bring together the classic economic concept of decreasing returns with the more recent notion of a declining EROI. The article also draws on work analysing how environmental activists use concepts from ecological economics, while at the same time ‘activist knowledge' contributes to ecological economics in a two-way communication between activism and science.