This error has been driving me INSANE!!!!! All these comments about workspaces and stuff are not the issue, there is something in arcmap that is screwing up.
FIX:
Bring in an editable layer (i.e. shapefile, you cannot edit your raster).
Start editing. And area solar radiation should work while you are in edit mode.
Be interested to hear if this is working for others.
I got my issues resolved using this method.
Originally I was getting the 999999 error trying the points to solar tool, once i tried it in editing mode it worked fine.
Then i stopped editing and tried after i disabled background processing it worked fine as well.
So it seems as if the background processing was the culprit!
I agree that not changing the default output location (ArcGIS default) does the trick.
The above suggested method of using the editing environment worked for me at first but then started giving the same error. But the default location trick is still working perfect for me.