The forecast by the U.S. Solar Energy Industries Association trade group and energy research firm Wood Mackenzie comes as the solar industry is lobbying lawmakers in Congress to pass an extension of the credit, which is worth 30 percent of the value of a solar energy system.

The incentive is scheduled to drop to 26 percent next year and decline annually before settling at a permanent 10 percent in 2022 for utility and commercial projects. Residential projects will lose the credit entirely after 2021.

The SEIA forecast assumes the tax credit is allowed to remain at 30 percent until 2030. Under that scenario, the United States would install 36 percent more solar energy than it would if the credit was phased out as scheduled. That additional 82 gigawatts (GW) of capacity would be enough to power more than 15 million homes, the forecast said.

More than three-quarters of the additional capacity would come from the utility sector, where solar increasingly competes on cost against other forms of energy.

The credit’s phase-out is a major change for an industry that has relied on it to underpin growth for well over a decade. Since it was implemented in 2006, U.S. solar installations have grown by more than 50 percent a year, according to SEIA. It has also helped create more than 200,000 jobs.

Democratic lawmakers in both the House and Senate have advocated an extension, but a key Republican, U.S. Senator Chuck Grassley, opposes it. The Senate Finance Committee chairman, a longtime supporter of the credit, has said he is against an extension because he promised opponents of the subsidy in 2015 – the last time it was extended – that he would not seek it again.