Bloomberg.com: Bloomberg Columnists: "Putting it all together, they found that it takes 29 percent more energy to make ethanol from corn than is contained in the ethanol itself.
It's not that corn is a bad source for ethanol. The other sources mentioned by the president look even worse. Wood biomass takes 57 percent more energy to produce than it contains. Switch grass takes about 50 percent more.
Ethanol is just a highly uneconomical product. Some other authors have disputed these findings, but they invariably come up with more favorable calculations by excluding some of the costs."