Nearly half of the solar power capacities under implementation, worth Rs 28,000 crore, face viability risk because of the continuous fall in the rupee, which has made imported solar modules costlier and increased the cost of setting up solar plants, Crisil said.

According to the rating agency, these Rs 28,000 crore worth of solar power capacities include 5,500 MW of projects bid out in the past nine months at very low tariffs of Rs 2.75 per unit or less.

These projects are in the early phase of implementation and unlikely to have bought solar modules, orders for which are typically placed 9-12 months after bids are won, it said.

According to the agency, solar modules account for 55-60 percent of the project cost of a solar plant, which is typically Rs 5 crore per MW.

"Today, over 90 percent of them are imported. Our analysis shows that for every 10 percent drop in the rupee, the cost of setting up a solar power plant increases by Rs 30 lakh per MW, assuming other factors remain unchanged," said Subodh Rai, senior director, Crisil Ratings.

The rating agency further noted that developers typically do not hedge the exchange rate before placing orders for modules and therefore, what was anticipated for bidding at low tariffs and has also worked for the developers is the fall in module prices.

The module prices have fallen by 17 percent for these projects from 0.30 dollar per watt at the time of their bidding to around $0.25 per unit at present, a benefit of nearly Rs 34 lakh per MW, it said.

"But the arithmetic did not countenance a sharp depreciation in the rupee to more than Rs 73 per dollar, which has wiped off the gains from lower module prices. That, in turn, will compress the debt servicing cushion available for these projects," the agency added.

For future projects, another risk according to Crisil is the levy of safeguard duty on imported solar modules of 15-25 percent for two years, with effect from July 30.

Apart from this, the decision to implement the duty will depend on the verdict of the Odisha High Court, which is hearing a petition by developers, it said.

"If the rupee remains weak and safeguard duty is also levied, project costs would dart up by as much as 20 percent. In such a situation, viable tariff for future projects will have to be higher by 30 paise per unit," said Manish Gupta, director, Crisil Ratings.

He noted that this would impact the government's target of setting up 100 GW solar capacity by FY22 because discoms would balk at buying renewable power at higher tariffs.

Moreover, Gupta said, lenders could also turn cautious to finance projects at a tariff of less than Rs 3 per unit due to their slim debt service parameters.

However, he said financially strong developers may be able to manage the risk to some extent by prudently funding projects with lower external debt component and bringing efficiencies in the operation and maintenance cost per MW because of scale and ability to negotiate with vendors.