What: It hasn't been a very sunny month for shareholders of Vivint Solar Inc (NYSE:VSLR). The stock is down 22% over the past month along with many other solar stocks.

Part of the sell-off in solar stocks was due to the market's move late in the month and a slow decline in the price of oil. But this move is more notable than most because Vivint Solar has increasing competition, not for customers, but for investors' attention in the solar industry. 

VSLR Chart

VSLR data by YCharts

So what: Some of the sell-off of Vivint Solar followed other solar stocks, which fell late in June and early July as oil prices dropped. But the stock also fell notably after Sunrun, a competitor in the residential solar space, filed initial IPO paperwork.

Sunrun is larger overall than Vivint Solar, although Vivint has grown so quickly that it's now installing more solar per quarter than Sunrun. But the addition of another solar player to the public markets will have the two companies competing for investors' dollars.

Now what: There's nothing fundamental to be worried about here and we saw a similar reaction in SolarCity's stock when Vivint Solar filed for an IPO. What many solar investors are likely doing is earmarking some of their renewable energy dollars to this IPO, selling Vivint Solar's stock in the process.

But I would argue that Vivint Solar is on more stable financial footing than Sunrun and is growing more quickly as well. Traders might be distracted by the shiny new IPO right now but Vivint Solar is still in good shape in the residential solar market and this discount is a nice buying opportunity for investors.