I think most of your doubts about my calculations come from the fact that I did not provide references. This is really a bit of a murky area since sources of information on the solar spectrum tend to vary in usefulness. For example the source below gives the solar spectrum in terms of single-photon counts for 0.010 nanometer wavelength increments. It would take a hundred years to get anything meaningful from this, but here it is:

High Resolution Solar Spectrum

Other sources are more meaningful. I used this one.

Solar Spectrum, Variability, Atmospheric Absorption

Here's another one that explains clearly that the solar irradiance measurements are indeed space-based, and not for ground-level. So I am absolutely certain that I got the solar irradiance for low-earth orbit correct:

http://www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/irrad.html

For ground level the solar irradiance varies with the season and it is even harder to find reliable information. In fact you may be correct that I am greatly underestimating ground-level sunlight.

Yes in fact I think I've screwed up ground-level sunlight. I will look closer at this. I don't anticipate major changes with my essays, but I will let you know when the changes are made and where to find them.