In India about 70% of overall electricity generation is based upon thermal energy. Huge amounts of non renewable sources such as coal are being consumed for the production of thermal power which leads us to degradation of the environment. Hence it is imperative to shift to renewable sources like solar energy for the generation of electricity. At present scenario generation using solar power is perceived be too costly. Installation of solar equipment for the typical house hold is difficult as enough technical awareness is lacking in choosing suitable components with appropriate ratings. In this paper a mathematical model is suggested, which can perform techno-economical analysis of off-grid rooftop solar system. Two detailed illustrative examples are mentioned using the realistic load specifications and cost of solar technologies available in Indian markets. For a typical hostel building having 192 rooms in 6-floors, the cost of rooftop solar PV system is about Rs. 1.33 Crores, with a life time profit of Rs. 1.95 Crores (in 25 years). The payback period is 10 years with reduction in CO2 emissions of 4490 tons. This paper although using a conservative model, is based on prevailing market conditions, makes a compelling case for consumers shifting from utility grid to rooftop solar PV systems. © 2016 IEEE.