Proportion of solar energy falling on earth: 1 / (2.21 * 10^9) 
Solar energy falling on earth: 1.6 * 10^17W
Earth surface area: 5.1 * 10^14 m^2
Average energy per unit surface area: 313W / m^2
A proportion of that energy is reflected or absorbed into the
atmosphere before it reaches ground level (Earth overall albedo is
about 30% ).
Also not sure how much of that energy is realistically useable, e.g.
what the spread of energy is over the EM spectrum and how that fits
with our ability to use the energy (e.g. what is the EM utilisation
profile of solar techs?)
2008 world energy consumption rate: 15 terawatts (1.504 * 10^13W)
, so about 1/10000th the solar energy hitting the earth.