EROI
Energy Return on Investment: the ratio of energy produced by a system to the energy invested in its construction and operation.
Formula
EROI = Total Energy Output ÷ Total Energy Input
Real World
Tar sands extraction in Alberta, Canada has an EROI of roughly 3:1 — meaning three units of energy are produced for every one spent — compared to conventional Saudi Arabian oil fields in the 1930s which yielded around 100:1, illustrating how declining EROI makes extraction increasingly uneconomical.
Exam Focus
Always express EROI as a ratio (e.g. 20:1) and clarify that values below ~3:1 are generally considered economically unviable.
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