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 exergy) delivered from a particular energy resource to the amount of exergy used to obtain that energy resource.
Arithmetically the EROI can be defined as:
When the EROI 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 ...