The region in which the rooftop or the ground mounted solar power plant is located is by far the most important factor that determines solar power output.
The location determines the DNI (Direct Normal Irradiance), which is the measure of solar energy falling per sq.m per day at that location. The higher the DNI, the higher the electricity produced by a solar cell.
DNI is measured in kWh/m2/day, that is, it represents the amount of energy that falls in a square meter in a day.
You can get approximate values for solar radiation data for particular locations from the following NASA website: https://eosweb.larc.nasa.gov/sse/RETScreen/
India is not just a large country; it is a country with diverse regions having significantly different climate and sunshine patterns. Thus, while a good portion of the country receives excellent sunshine most part of the year, there are regions that are not fortunate enough. The sample data in the following table will show you why.
A quick analysis of the above table will confirm what you would have suspected already – as you start going east of West Bengal, the solar power plant starts generating far less. Conversely, the west (Rajasthan & Gujarat especially) bask in sunshine most parts of the year and generate some of the highest amounts of electricity.
The surprise package in the table of course is Leh in Kashmir, but in fact it should not be a surprise at all. You see, Leh not only gets excellent sunshine, it is also a cool place – DNI being the same, solar panels generate a bit more electricity in cooler climates than in hotter ones.