Energy has played a critical role throughout human society's demographic, economic and social development. The availability and quality of various energy and material resources to a society is linked to the general trend of the settlement, growth, and eventual decline experienced by each civilization. A society must have an energy surplus for there to be division of labor, creation of specialists and the growth of cities, and substantially greater surplus for there to be wide-spread wealth, art, culture and other social amenities. In energy economics and ecological energetics, energy return on investment (EROI), also sometimes called energy returned on energy invested (ERoEI), is the ratio of the amount of usable energy (the energy) delivered from a particular energy resource to the amount of energy used to obtain that energy resource.When the EROEI of a source of energy is less than or equal to one, that energy source becomes a net "energy sink", and can no longer be used as a source of energy, but depending on the system might be useful for energy storage (for example a battery). A related measure Energy Stored On Energy Invested (ESOEI) is used to analyse storage systems.