In physics, energy economics and ecological energetics, EROEI (Energy Returned on Energy Invested), ERoEI, EROI (Energy Return On Investment) or less frequently, eMergy, is the ratio of the amount of usable energy acquired from a particular energy resource to the amount of energy expended to obtain that energy resource. When the EROEI of a resource is equal to or lower than 1, that energy source becomes an "energy sink", and can no longer be used as a primary source of energy. » EROEI = frac ) +… (More on Eroei)