The literature is replete with discussions about the sustainability of our current energy technology. There seems to be no clear agreement, however, about what sustainability means. In this paper we attempt to provide precise criteria that define the sustainability in a future economy that is based solely on alternative energy sources. At present, alternative energy contributes only a small fraction to the total energy we consume. However, in the future, the energy contribution from alternative sources will become more and more significant; it is therefore important to investigate whether or not such sources would be sustainable or not. The heavy subsidization required for the full development of alternative energy sources justifies an investigation into the extent to which they can really contribute to the production of energy. When this same question was raised in the 1970s, it was called “net energy”, which is not the same as sustainability. For example, some claim that producing ethanol from corn results in 25% more energy than the oil, natural gas and electricity used in the entire process. We will show, however, that if our economy would be based only on alternative energy sources, the hydrocarbons obtained from ethanol would be completely consumed in the production of more ethanol: this is not a sustainable system. One should look, therefore, at the total system when assessing the sustainability of any technology, especially if it has to play a major role in an economy based only on alternative energy. Our paper presents a systematic, but simple, estimation procedure. When ethanol from corn is considered as an example, the energy consumption of the entire sector must be taken into account (e.g., fertilizers, irrigation, pesticides, herbicides, lime, transportation, production of needed equipment, conversion and distillation processes, etc.), but also the energy consumption of all those who are involved in ethanol production, to be certain that much more energy is produced than consumed. Despite the disparity in the results obtained when different research groups calculate the net energy from ethanol, we will demonstrate that, correctly interpreted, all the published data shows that “ethanol from corn” is not a sustainable technology. Furthermore, although ethanol is used as an example because it is widely discussed, we will show how this analysis applies to other sources of alternative energy, such as solar thermal and hydrogen.