Update: Surface Area Required to Power the Whole World With Solar and Wind Power
"Solar power systems installed in the areas defined by the dark disks could provide a little more than the world's current total primary energy demand (assuming a conversion efficiency of 8%). That is, all energy currently consumed, including heat, electricity, fossil fuels, etc., would be produced in the form of electricity by solar cells. The colors in the map show the local solar irradiance averaged over three years from 1991 to 1993 (24 hours a day) taking into account the cloud coverage available from weather satellites." Note that current solar panels have an efficiency higher than 8% (more than double that, in many cases). For more info and details on the sources, see this page. Via Reddit. Thanks to Matthias Loster!