Nerd 1: (Looks at watch. 4:45 PM) What do you think? 600 watts per square meter?
Nerd 2: (Looks at sky.) Given the altitude, perhaps 650.
Nerd 2 works at the Department of Energy's Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) Central Facility, Oklahoma so she looks at solar irradiance data for a living. Nerd 1 has no excuse other than being a nerd.
A quick search yielded a compendium of solar irradiance data servers put together by NREL. Following the link to National Wind Technology Center, enter in the day of the hike and voila. Click to make the graph below bigger and you will note that, taking into effect daylight savings time and averaging over the noise of passing clouds, Nerd 2 was spot on!
BTW, solar irradiance is a measure of the amount of light energy per unit area from the sun, integrated over all light frequencies. See the solar spectrum and images of the sun at different frequencies, including the UV and X-rays. Fortunately, the ozone layer screens out much of the UV so we don't fry here on the surface.
If you want to see how much power you can generate with photovoltaic cells on your house, take a look at the climatology of solar irradiance near you and multiply by the efficiency factor for the type of panel (a number much less than 1 or 100% efficiency). Does the sun generally shine when you need to draw power?
The reliability and availability of solar power deserves it's own post. Later.