A 2015 review in Renewable and Sustainable Energy Reviews assessed the energy payback time and EROI of solar photovoltaics.In this study, which uses an insolation of 1700/k Wh/m²/yr and a system lifetime of 30 years, mean harmonized EROIs between 8.7 and 34.2 were found.
EROEI and Net energy (gain) measure the same quality of an energy source or sink in numerically different ways.Net energy describes the amounts, while EROEI measures the ratio or efficiency of the process.They are related simply by For example, given a process with an EROEI of 5, expending 1 unit of energy yields a net energy gain of 4 units.The break-even point happens with an EROEI of 1 or a net energy gain of 0.In physics, energy economics, and ecological energetics, energy returned on energy invested (EROEI or ERo EI); or energy return on investment (EROI), is the ratio of the amount of usable energy delivered from a particular energy resource to the amount of energy used to obtain that energy resource.It is a distinct measure from energy efficiency as it does not measure the primary energy inputs to the system, only usable energy.
A fuel or energy must have an EROEI ratio of at least 3:1 to be considered viable as a prominent fuel or energy source.
When the EROEI of a resource 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 Store On Energy Invested (ESOEI) is used to analyse storage systems.
The natural or primary energy sources are not usually included in the calculation of energy invested, only the human-applied sources.
For example, in the case of biofuels the solar insolation driving photosynthesis is not included, and the energy used in the stellar synthesis of fissile elements is not included for nuclear fission.
The energy returned includes usable energy and not wastes such as heat, although depending on source and application, waste heat is used in district heating and water desalination, these cogeneration plants however are rare, globally, and thus usually excluded in EROEI analysis of energy sources.