In recent years, there has been much discussion about the energy balance of certain biofuels, particularly corn ethanol, cellulosic ethanol, and biodiesel. Scientists have attempted to establish, using life cycle analyses (LCA), the merits of these fuels–most importantly, their “energy balance” or “energy return on investment” and lifecycle greenhouse gas emissions reduction.
The energy balance of a product is defined as the energy content of the product minus the sum of all energy inputs required to produce the product. For example, the process to convert one bushel of corn into ethanol requires direct inputs of heat and electricity. However, it also requires secondary inputs such as diesel fuel to drive the tractor to harvest the corn, as well as energy to build the tractor. These analyses get very complicated, and it is not clear where the “system boundary” should lie. Does the fuel for the tractor count as an input? Most definitely. What about the energy content in the ham sandwich the hapless farmer ate for lunch? Maybe–but he has to eat whether or not he is harvesting corn. Should we count the energy for the tractor to harvest the wheat to make the sandwich? Should we be doing energy balances on ham sandwiches? And if the energy in a ham sandwich is less than the sum of the energy inputs, should we stop making ham sandwiches?
Obviously the answer is no. Ham sandwiches are delicious, and I like eating them. I also like putting mayonnaise and mustard on my sandwiches, condiments with abysmally low energy content. And, of course, drive to the grocery store to buy these things.
The upshot of all this is that people do not use energy–they use heat, light, mobility, and consumable products (like food and toilet paper), all of which require energy inputs. Thus the energy balance is useful for determining how some goods compare with others, just like price. It would be useful to compare the price of two supplementary goods (such as gasoline and ethanol) in deciding which one to purchase. But it would be strange to arbitrarily decide that if a fuel costs more than $1/mile, then it is not an efficient option.
Critics of certain biofuels will often point to the energy balance calculation as the “smoking gun” that these fuels are inefficient and a waste of energy–or indeed, an evil plot perpetrated by the farm lobby, big business, or big government. What they instead should do is compare the energy balance of ethanol with that of gasoline, a common substitute. These same critics might be surprised to find out that it requires 1.19MJ of fossil energy to produce 1MJ of gasoline , resulting in a negative energy balance. Corn ethanol, in comparison requires only .77MJ of fossil energy for 1MJ of corn ethanol , an improvement of 54%. The energy balance calculation is therefore best viewed in a relative, rather than absolute sense to avoid confusion.
 Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones, A. D.; O’Hare, M.; Kammen, D. M. Ethanol can contribute to energy and environmental goals. Science. 2006, 311, 506–508.