Revised 24 May 2019

The energy return on energy invested (EROEI or ERoEI) or energy return on investment (EROI) is the ratio of the amount of usable energy (exergy) delivered from a particular energy source to the amount of exergy used to obtain that energy resource. 

EROI  =  Exergy Delivered / Exergy Required For Delivery

If the EROI of an energy source is less than or equal to 1, then the energy source is an “energy sink” and not a sustainable energy source. Although it takes more exergy to deliver hydrogen than the exergy of the delivered hydrogen, it might be useful in some cases to use hydrogen as a carrier of energy and likewise with the storage of energy in a battery. The Energy Store on Energy Invested (ESORI) is used to analyse energy storage systems. Comparisons of the EROI of different renewable energy sources and the ESORI of different storage systems are addressed in the Chapter: Issues of Sustainability