In 2005, the overall efficiency of global energy conversion (from primary energy to services) was about 11 per cent (Cullen and Allwood, 2010a). In other words, global primary energy demand could be reduced to only one ninth, while the same energy services were provided, if all energy conversion devices were operated at their theoretical maximum efficiency. In more practical terms, but still assuming an almost perfect world, global primary energy demand could be reduced by 73 per cent (or to less than one fourth), while the current level of energy services were provided, mainly through a shift to passive systems (Cullen, Allwood and Borgstein, 2011). This is in line with the popularized overall “factor 4” and “factor 5” improvements (von Weizsacker, Lovins and Lovins, 1998).