If you want to quantify available solar, there are two problems: 1) incident at the upper atmosphere not accounting for any cloudiness or haze and 2) how much makes it through to the surface at your solar collectors. The first is easy and the second is a lot more complicated. I cover this topic in two books, showing actual data. People assume the sky is clear, but this is often not the case. It's only *technically* clear in Phoenix, AZ (a place known for "clear" skies) about 27% of the time, which is why solar power plants rarely achieve their expected capacity. NREL has a formulation (by Reda and Andreas) that will yield solar position for anywhere on the earth, but this is unnecessarily complicated. CIEMAT has a much simpler algorithm that should be adequate. You can download it at http://www.psa.es/sdg/archive/SunPos.cpp You can also find the formula in an Excel spreadsheet (solar_position.xls) in folder examples\solar inside the archive that accompanies the text where I explain it. The Phoenix data is in the same folder. http://dudleybenton.altervista.org/software/Simulation.zip