Radiation data is available from various sources, such as IMD, NREL, Meteonorm, NASA, WRDC (World Radiation Data Centre) and so on. Some of these agencies provide data free of cost and with others, the data needs to be purchased.
Meteonorm provides data of more than 8,055 weather stations. The measured parameters are monthly means of global radiation, temperature, humidity, precipitation, days with precipitation, wind speed and direction, sunshine duration. Time periods 1961-90 and 1996-2005 for temperature, humidity, precipitation and wind speed are available. Satellite data is used for areas with low density of weather stations. Interpolation models are provided in the software to calculate mean values for any site in the world. The user may import data for use in the models. This data is not freely available, and must be purchased along with the Meteonorm software. For further details refer http://meteonorm.com/
WRDC (World Radiation Data Center) provides monthly irradiance for 1195 sites in the world, averaged during periods between 1964 and 1993. Many of them are only over a few years. These data doesn’t include temperatures, which should be obtained from another source. This data is available free of cost. For further details log on to http://wrdc-mgo.nrel.gov/
RETScreen is Canadian software which holds a complete database for any location in the world, optimised for using the best available data at each location from about 20 sources, the main ones being the WRDC and the NASA irradiance data. Temperatures and wind velocities are also provided probably with good reliability. NASA and WRDC data are available free of cost, and hence RETScreen data is also free and can be downloaded from http://www.retscreen.net/ .
IMD (Indian Meterological Department)
IMD has 45 radiation observatories recording various radiation parameters. At all these stations, measurement of global solar radiation is being carried out while at a few selected stations other parameters like diffuse, direct, net, net-terrestrial and reflected radiation and atmospheric turbidity are also measured. Data loggers have been introduced at four stations viz. New Delhi, Patna, Jaipur and Thiruvanathapuram. this is a paid data service. For further details log on to http://www.imd.gov.in/
NASA provides over 200 satellite-derived meteorology and solar energy parameters. These are monthly averages from 22 years of data. Global solar energy data is available for 1195 ground sites. These data are available free of cost. For further details log on to http://nssdc.gsfc.nasa.gov/