New Delhi, Oct. 5 -- On the terrace of MJR Pearl, a 14-storey apartment complex in Bengaluru's Whitefield, rows of photovoltaic panels have turned the roof into a 150-kW power plant. For the residents of the 416-apartment complex, installing them was a leap of faith.

In most high-rise housing societies, rooftop solar proposals run into questions about the initial investment, the time it will take to recover the cost and whether the savings will justify the expenditure. At MJR Pearl, the decision has paid off; the installation has cut the complex's common-area electricity bills by 80%, bringing down monthly maintenance costs.

Across Indian cities, more and more rooftops are housing solar plants. Central government subsidies, additional s...