The Atmospheric Science Data Center (ASDC) at NASA Langley Research Center offers several data sources. For example, it is possible to download a text file with the 22-year (July 1983 – June 2005) monthly and annual average of global horizontal irradiation.
nasafile <- 'http://eosweb.larc.nasa.gov/sse/global/text/global_radiation' nasa <- read.table(file=nasafile, skip=13, header=TRUE)
With this data, R and the solaR package we can calculate (for example) the global effective irradiation incident on several surfaces: a fixed PV generator, a two-axis tracker and a N-S horizontal tracker. First, let’s plot the original data using some spatial packages: