Energy Returned on Energy Invested (ERoEI) And Why It Matters

Energy Returned on Energy Invested (ERoEI) – also sometimes referred to as EROI — is a key energy accounting metric that measures the net usable energy that can be obtained from some potential energy source after all of the various energy costs necessary to produce usable energy from the potential energy source have been subtracted from the estimated life time energy production of the potential source.

It is rapidly becoming a rather widely quoted statistic, but remains poorly understood by many people who are starting to use it or are becoming exposed to it for the first time. ERoEI is deceptively simple and seems very straightforward, but it masks a complex underlying computation that is subject to some important and arbitrary decisions. These underlying assumptions and boundary decisions have profound effects on the resulting ERoEI figure. For this reason, if for no other, it is important that as wide an audience as possible become more educated about what ERoEI means and how the many assumptions that go into a particular ERoEI calculation can lead to significantly different ERoEI figures for the same energy system.