Image source: SolarCity.

What: Shares of solar installer Sunrun Inc. (NASDAQ:RUN) plunged 19.5% in May as the company faced a lot of problems from slowing growth.

So what: Like SolarCity, Sunrun reported first-quarter 2016 results that showed a big slowdown in bookings growth and a sharp rise in costs, which isn't a good sign for the rooftop solar business. There's little urgency for customers to go solar after the solar investment tax credit was extended and now there's an emerging threat from loans that are helping smaller, regional installers. No one also quite knows how these negative trends will play out throughout 2016.

Now what: Rooftop solar faces a lot of challenges from a variety of larger energy trends right now. Loans threaten Sunrun's business model, slowing growth means higher costs for sales and operations, and borrowing costs are climbing as well. That doesn't bode well for Sunrun, and with little technical differentiation to offer customers, higher costs than competitors, and a reliance on leases and power purchase agreements, which are losing market share, I don't see a bright future.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.