Energy Return on Energy Invested

Energy Return on Energy Invested (EROI) is the ratio of the usable energy obtained from an energy source to the energy consumed in making that energy available. It indicates the source's ability to provide net energy to society, rather than merely producing gross energy output.

How the ratio is interpreted

EROI is calculated as energy returned divided by energy invested. An EROI of 10:1 means that ten units of energy are delivered for every unit used, leaving a net energy surplus of nine units.

• An EROI greater than 1:1 produces a positive net energy surplus, while a value at or below this level provides no net energetic gain.
• A higher EROI generally means that a smaller share of society's energy must be reinvested in obtaining energy.

Measurement and comparison

The result depends heavily on the chosen system boundary and on which energy inputs are counted. Meaningful comparison therefore requires consistent assumptions across energy sources.

• Inputs may include energy used in extraction or cultivation, processing, transport and infrastructure, depending on the study's boundary.
• EROI differs from conversion efficiency, which measures the output-input relationship within a particular conversion process.
• EROI is an energy metric, not a measure of financial profitability or market price.

Relevance to ethanol sustainability

For ethanol, an EROI assessment compares the fuel's energy content with energy used in feedstock cultivation, irrigation, fertiliser production, harvesting, transport and conversion. Treatment of co-products can materially affect the calculated ratio.

• A favourable EROI alone does not establish overall sustainability because it does not directly measure water depletion, land-use effects, biodiversity impacts or life-cycle greenhouse-gas emissions.
• EROI should therefore be used alongside water, emissions and broader life-cycle assessments when evaluating biofuels.

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