Renewable energy lobbyists on Wednesday held a press conference to warn that the failure of Congress to extend a key financial incentive would be disastrous for the solar and wind industries. At the same time, IDC, a research firm, released a report predicting that the North American photovoltaic market will double in 2011.

So what gives?

What has the renewable energy industry worried is the expiration at year’s end of a Treasury program that lets developers take a cash grant to cover 30 percent of the cost of big solar, wind and geothermal projects in lieu of taking an existing investment tax credit.

Since most green energy companies have no profits to offset with the tax credit, the cash grant has become crucial to obtain financing to build multibillion-dollar solar power plants and wind farms.

“It’s simply the most important policy for continuing growth of renewable energy in the United States,” Rhone Resch, president of the Solar Energy Industries Association, said Wednesday during a press conference.

While some of those projects are photovoltaic farms that install tens of thousands of solar panels like those found on residential roofs or on the ground in huge arrays, many are solar thermal power plants that use mirrors to heat liquids to create steam to drive an electricity-generating turbine.

While the tax credit and federal loan guarantees are critical for such utility-scale projects, the residential and commercial rooftop market depend on a host of state and federal incentives that won’t be as affected if the cash grant program disappears.

California, for instance, runs a program that is subsidizing the installation of 3,000 megawatts of rooftop solar while states such as Oregon, Illinois and Massachusetts have carved out quotas for photovoltaic projects in their renewable energy targets.

Hence, IDC estimates that installed photovoltaic capacity in North American in 2011 will more than double to between 2.3 gigawatts and 2.7 gigawatts.

“While subsidies undeniably underpin much of this growth, the young solar PV industry has been rapidly innovating in ways that make solar PV systems more cost effective, easier to install (from both an economic and an installation labor perspective), and easier to maintain,” the report states.

Contributing to the growing rooftop solar market has been a rapid decline in photovoltaic module prices over the past two years, increased competition with low-cost Chinese manufacturers and the spread of leasing programs that allow homeowners to go solar without incurring the significant capital costs of installing photovoltaic arrays.

(Photo: Todd Woody)