I am looking to get current and historical sun radiation or illuminance data for given locations around the globe. I've had a look at the wunderground API as I also need generic weather data but it appears that it doesn't include that info even though some weather stations do provide it. The ticket requesting it is 1 year old and doesn't seem to have moved very much in that time so it doesn't give me confidence that this will be made available any time soon. Does anybody know of any good alternative source for such data?
Note that in terms of location, I would prefer an API that has a worldwide cover. Failing that, my highest priority is UK data followed by Europe and then the rest of the world.