EROEI

EROEI, or energy return on energy invested, is a metric used to assess the efficiency of an energy-producing resource. It expresses the ratio of usable energy obtained to the energy expended to obtain, process, and deliver that energy. The result is typically given as a ratio, such as 10:1, meaning ten units of energy are produced for every unit invested.

Calculation depends on system boundaries. The energy invested may include exploration, extraction, refining, processing, construction of infrastructure, maintenance, and decommissioning. The energy delivered is the usable energy that can power end-use services. Results can be affected by the time horizon of the analysis and by whether energy quality (exergy) is considered.

Estimates vary widely by resource and methodology. Conventional oil historically had high EROEI, but declined over time; modern estimates commonly place it roughly between 5:1 and 20:1. Unconventional sources such as oil sands tend to be lower. Renewable sources show broad variation: solar photovoltaic estimates often range from about 6:1 to 12:1, wind from 20:1 to 30:1, and hydropower generally on the higher end; nuclear results depend on assumptions.

Limitations include that EROEI does not directly capture price, subsidies, environmental impacts, reliability, or energy quality differences. It also depends on boundary choices and time factors. Thus, EROEI is a useful but imperfect tool in energy planning, best used alongside economic and environmental assessments.

In practice, it is used in energy economics and policy discussions to compare long-term sustainability of energy options.