In his recent book The Upside of Down, a review of which can be found here, Thomas Homer-Dixon interpreted the development of the Roman Empire in terms of thermodynamics. The success of the empire depended on its ability to extract energy surpluses, in the form of food, from the imperial territories and concentrate them at the centre, where they enabled the development of a tremendous degree of organizational complexity. Without a large, and growing, hinterland to collect surpluses from, complexity on such as scale would not have been possible to establish and maintain.

But wherever the farms were located, they played a role in the Roman energy economy similar to that of solar battery chargers: they converted sunlight into a form of high-quality potential energy, especially fodder and grain, that was storable and transportable.

The Romans then focused this energy – they used their food batteries, so to speak – to create a productive, resilient, and phenomenally complex system of public buildings, manufacturing facilities, housing, roads, aqueducts, and social organization.