Similar to the answer for the question on payback period, the answer to this question actually is: It depends.
The returns you make on solar power plants (as measured by the various financial yardsticks such as equity IRR or project IRR) depend on the financial incentives generated from solar power. These incentives can take two forms: Savings if solar is for captive consumption, or revenues if you are selling solar power to the government or a private third party firm.
The savings in turn depend on not just the cost of solar power, but also the current price you are paying for grid power, which varies from one state to another, and from one sector to another. A manufacturing company in Tamil Nadu could be paying about Rs 6.5/unit to the electricity company, while a hotel in Maharashtra could be paying Rs 13/unit!
Similarly, the revenues you make from selling solar power to government or third party private firm also vary significantly from one case to another – in 2016 & 2017 alone, we have seen tariffs ranging from as low as Rs 3/unit to Rs 6/unit!
It is thus obvious that there is no one number we can indicate for rates of return.
However, we do not wish to leave you with nothing! Here are some benchmarks you can take with you.
For MW-scale ground mounted solar power plants in which the power is sold to the government utilities, one can expect project IRRs of about 10-12% for a 25 year project period and equity IRRs of about 15%. These IRRs could be 2-4% higher where the power generated is sold to private firms, and could be a lot higher when it is used for captive consumption. (Captive consumption leads to higher IRRs as it usually offsets a high tariff)