What happened 

Shares of residential solar company Vivint Solar (VSLR) jumped 15.2% in January, according to data provided by S&P Global Market Intelligence, as sentiment for the solar industry turned bullish. 

So what 

Early in the month, Goldman Sachs analyst Brian Lee reiterated a buy rating on Vivint Solar's stock, although he lowered his price target from $7.50 to $6.50. After a rough 2018 that included an escalating trade war with China and falling installations, Lee thinks solar demand will pick up in 2019. 

Check out the latest Vivint Solar earnings call transcript.

Solar panels on top of a house's roof against the shining sun.

Image source: Getty Images.

Two of the other big factors driving solar stocks higher are falling interest rates and the market's expectation that rates may rise slower than previously forecast. Since Vivint Solar finances most of its solar systems, lower interest rates mean higher project values and more value on the company's balance sheet. 

10 Year Treasury Rate Chart

10-year Treasury rate. Data by YCharts.

Now what 

Solar stocks have been up and down for the past year, and the start of 2019 has been no different. What drove Vivint Solar higher, in particular, were low interest rates and speculation that this year would see a return to growth. That's the hope, but we don't yet know if that bullish thesis will play out. 

Vivint Solar will report fourth-quarter 2018 results after the market closes on March 5, 2019, and investors will learn more about residential solar demand at the end of 2018 and expectations for 2019. What management sees going forward is more important for the long term than the market's ups and downs in any given month.