The aim of this article is to analyse the physical and ecological limits of biofuels (in particular, ethanol). To this end, three aspects are discussed. First of all, the territorial element, i.e., the real productivity of energy crops (in particular, ethanol), to show that, in general, the productivity is much lower than most of the literature on the subject suggests. Similarly, we then stress the high total territorial impact (the Ecological Footprint) associated with this kind of energy production. Thirdly, we offer data concerning the very modest nature of the energy balance of biofuels (EROEI) and their real power density, considering the productivity of the crops grown for biofuel. As a comparison, these three aspects are estimated for other forms of energy, in particular, photovoltaic energy. We conclude that when the set of estimated parameters has been analysed, there exist reasonable doubts concerning the use of biofuels on a regional and global scale, so they should not, in principle, be promoted as a renewable energy source, nor is it desirable on such a scale.