The third quarter of 2017 has been an unusually slow one for the solar industry, according to a report from GTM Research released on Thursday. US solar installations fell to their lowest overall level since 2015, and residential and utility-scale solar projects fell quarter over quarter.

The only silver lining has been non-residential (largely commercial and community) solar installations, driven by developers in California, New York, and Massachusetts rushing to take advantage of state-level incentives, as well as installations in Minnesota boosted by Xcel Energy's "community solar" program

Overall, solar panel installations in Q3 decreased 51 percent from Q3 in 2016, says GTM Research.

Residential photovoltaic installations slowed 10 percent, quarter over quarter. According to Reuters, some of this slowdown may in fact be due to Tesla's purchase of SolarCity in late 2016. Tesla announced earlier this year that it would cease SolarCity's aggressive door-to-door sales tactics and focus on selling solar panels through Tesla showrooms. Reuters notes that in Q3, solar installations from Tesla were off 42 percent from the previous year.

Tesla declined to comment to Ars but pointed to its Q2 2017 financial statement. "Rather than prioritizing the growth of MW of solar deployed at any cost, we are selectively deploying projects that have higher margin and generate cash up front," the company wrote. "Consequently, solar energy generation deployments in Q1 2017 declined year-over-year, but had better financial results. Furthermore, the portion of residential customers who elected to purchase rather than lease a solar system grew to 31 percent of deployments in Q1, up from 9 percent a year ago, improving the cash generation of this business."

According to Reuters, SolarCity accounted for 25 percent of the national residential solar panel installations in 2016, but, by Q3 this year, its share of installations only represented 14 percent of the panels in the national market. That has given an opening to smaller competitors like Sunrun, which expects to see installations rise by 15 percent this year.

Still, news isn't bad for the solar industry in a relative sense. In Q3, 2,031 MW of solar capacity were built—that's 100MW more than were built in all of 2011. Some of the slowdown on the utility side may also be a reflection of a slowdown in energy investing in 2016 as a whole, followed by a pretty stagnant 2017 for renewable energy investing numbers rather than a move away from solar. In fact, GTM Research notes, solar reflected 25 percent of US energy capacity expansion in the first three quarters of 2017, beaten only by natural gas additions.

As investors gear up for next year, uncertainty lingers. In September, two solar panel makers won a key decision from the International Trade Commission (ITC), which found that cheap imported solar equipment has harmed domestic manufacturers. The ITC recommended three different tariff schedules, but President Trump will ultimately decide what kind of tariff to levy on solar imports. Solar installers have said that any kind of tariff on cheap solar imports will hurt installation rates.