If EROI is high, the animal is efficient in obtaining what it needs to survive. If EROI is low, the animal is inefficient, and susceptible to harm in adverse circumstances.
By analogy, the same is true of societies. By maximizing the efficiency with which we obtain energy, we have more time, capital, and energy available to produce food, build housing, and manufacture goods. If we are inefficient in how we obtain energy, we have less time, capital, and energy available for those things.
High numbers are highly efficient: in the 1930’s, spending one barrel of oil (energy equivalent) to drill an oil well returned the investment 100-fold. That’s why oil millionaires (e.g. Jed Clampett, Fred Astaire’s “Daddy Long-legs”, J.R. Ewing, and real millionaires, such as the Hunt and Koch brothers) became part of our folk culture. However, exploration maturity and the law of diminishing returns nibbled away at the excess return. By the 1970’s, domestic oil had about 30-fold return on investment, and currently, the number is less than 10. (Keep in mind that there are other financial costs; this is just the return of energy produced compared to energy invested.)
Biofuels deserve special mention. Ethanol production from corn is supported by Federal subsidies, with are estimated to have cost $17 Billion from 2007 to 2010, and will cost $53 Billion by 2015, if the subsidies and mandated volumes are not repealed earlier. In theory, corn ethanol contributes to American energy independence and reduces carbon emissions. But, does it really? Estimates for the EROI of corn ethanol are very weak, ranging from a maximum of 1.6 to a minimum of 0.9. (Dr. Hall comments "....as compared to real fuels, which have EROI of 30 or 40). If we assume the mean, about 1.35, it means that 135 gallons of fuel must be produced in order to deliver 35 gallons of fuel to society. And in doing so, producing corn ethanol removes food from global markets, raising food prices in a hungry world; consumes large amounts of water; causes soil erosion and degradation of water runoff. Corn ethanol is an inefficient fuel, and supporting corn ethanol is bad public policy.
Any alternative to oil and coal must be Efficient, Scalable, and Timely. 1) Alternatives must have a high EROI. This is really easy to recognize, because energy is a large component in every investment dollar. Phrased another way, alternative energy must be substantially profitable, without subsidy. 2) Alternative energy must be scalable, in order to provide meaningful contribution to total energy supply. 3)Alternative Energy must be Timely. It must be possible to implement the alternative before serious economic contraction begins, reducing the capital available to build the alternative.