"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).
(source: Matthias Loster)
so, why aren't we doing so?