What happened 

Shares of residential solar installer Sunrun (RUN 3.93%) jumped 36.3% in September, according to data provided by S&P Global Market Intelligence, after getting over a big hurdle in acquiring Vivint Solar (VSLR). It didn't hurt that renewable energy stocks overall had a great month as well. 

So what 

The biggest news from Sunrun itself was an announcement that the Justice Department had granted an early termination of an antitrust waiting period. This paves the way for Sunrun to acquire Vivint Solar, with the deal expected to close any day.

Home with solar panels on the roof.

Image source: Getty Images.

It can't be overlooked that renewable energy stocks overall had a great month in September as well. One big factor was presidential candidate Joe Biden improving in the polls against President Trump, which is seen as bullish for renewable energy stocks. Biden doesn't support the Green New Deal, but he's much more likely to implement policies favorable to renewable energy, and that helped shares across the industry last month. 

Now what 

If Sunrun is able to close on the Vivint Solar transaction as planned, it'll have a dominant position in the residential solar market. That will allow it to potentially raise prices and lower costs, expanding margins for the entire business. That's what investors are hopeful for, and more-favorable policies in 2021 would be the icing on the cake for solar stocks.