Variability and uncertainity in solar resource data

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The solar sites are often evaluated  only in terms of magnitude—how much solar radiations are available at a given site for a typical meteorological year. However there is a wide variation in the solar radiation data over time and this interannual variability is some time of the order of about 10% or even more.

The variability of the solar radiations lead to the uncertainity in the electricity generation expected at a given site and is a major concern to the financing institutions.  It is not only important to assess the energy yield for a single year, there is a requirement to have the energy yield assessment on long-term basis for which you are required to have the historical solar radiation data at least for 10 years for the site location. So that the trend can be forecasted to give the expected future energy generation for the banker. The data is not generally viable through the free data source. However some of the paid data sources such as 3tier or solar GIS provides this data on cost basis, which makes your detailed project report (DPR) as a bankable DPR. Following example show the historical data for some of the typical paid site done by us.

bankability of solar -2

In order to have a successful financing, a thorough and rigorous evaluation of the solar resource data is required to be modelled for the selected technological combination. The data should be not only for a typical meteorological year (TMY), but also the data should be procured for the solar radiation variation over a day, month, year, or the duration of the
project life, especially for projects with long PPAs.

Ideally, the most bankable solar radiation data is considered based on the data recorded from the site specific solar monitoring station that is well maintained and the  measurements taken over 20 years or longer. However, very few data sets of that duration are possible. Recently MNRE has initiated the data solar radiation monitoring stations at over 50 sites, however the length of the data is critical in order to avoid the interannual variability of the solar resource data.

Some of the available database for solar radiation are as follows:


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


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


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 .

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


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


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