Smil (2010b, p. 110) notes that “direct solar radiation is the only form of renewable energy whose total terrestrial flux far surpasses not only today’s demand for fossil fuels but also any level of global energy demand realistically imaginable in the twenty-first century”. Leaving aside unsuitable locations constituting about half of the worlds land area (those characterized by weak insolation or inaccessibility) about 470 ZJ are available. Yet, it would be technically possible to harness only a small fraction of this, and even less would be economically or politically acceptable. For example, the very ambitious Global Energy Assessment efficiency scenarios assume a techno-economic potential for solar PV, solar thermal and solar water heating of 2.6 ZJ.