I have tried to calculate solar radiation (whole year with monthly interval) for Portugal from an ESRI grid (27m DEM) in Arcgis10. It seems to work, however it takes much more than a week and in the meantime there was an energy failure. I use ArcToolbox/Spatial Analyst Tools/Area Solar Radiation.
I have cut the dem so as to have only the south of Portugal in the hope that the processing would be faster.
However, ArcGIs 10 crashes after a few minutes and I get the error:
"Background processing has encountered a serious error and is unable to continue. Contact ESRI".
I have also tried in Arcgis 9.2 and I get the same message.
It simply seems not to like the cut dem.
Best Answer
As I know, SAGA GIS has some tools to calculate this. But I did not try yet. However it is free and able to work with asc grids and many other formats, so you can try it. Cutting grids works fine in SAGA - I mostly use a polygon shapefile to cut the grid, but there are also some interactive tools.