A dearth of accurate systems to measure the suns irradiation especially for India has been an area of concern for developers, investors, as well as engineering organizations with EPC arms in solar. This void is being slowly filled by companies such as SolarGIS who have launched their maps for India, in August 2011 at the 5th Renewable Energy Expo.
The tutorials and video guide can be accessed at the below links
The company claims that according to IEA SHC Task 36 study, theirs is amongst the best performing database.
The report can be downloaded at:
The annexes to the report, with detailed analysis for each validation site can be downloaded at:
Further on the same topic, to improve understanding of the solar wealth of the nation, the Ministry of Natural and Renewable Energy MNRE for India commissioned a joint study of the northwest region with the National Renewable Energy Laboratory NREL from the USA, the report for which can be accessed here. The latest Geospatial Toolkits for NREL are also available.