EROI = Energy Return On Investment
(or: EROIE = Energy Return On Invested Energy)
The energy that you need to invest in order to get an amont of energy.
- Oil extraction. In the beginning of the 20:th century, the energy investments needed to use the energy of crude oil was estimated to 1:25, i.e. your gain was 25-fold. Today, the same figure is estimated (globally) to less than 10, in the lower 48 to less than three. Whe the figure goes down to 1, the oil ceases to be an energy souce. Tar sands are estimated to give an energy gain of about 1.5, at least the shallow ones.
- The Rabbit limit. If you live solely of rabbits, you alwys have to invest a cetain amount of energy in rabbit hunting. This amont can never be more than the energy content of a rabbit, since you will lose energy in the activity. You will starve to death, and faster the more rabbits you catch in that way.
The EROI concept is closely related to the more broad concept of YPE, Yield Per Effort