What: Shares of residential solar installer SolarCity Corp. (SCTY.DL) dropped as much as 31% in early trading Wednesday after reporting fourth-quarter 2015 earnings.

So what: The big number investors are focusing on today is SolarCity's continually slowing growth. In the third quarter of 2015, the company lowered its own installation guidance for the year from 920-1,000 MW to a range of 878-898 MW. But actual installations were just 870 MW for the year. First-quarter installation guidance was for just 180 MW of installs, growth of just 18% year over year, so the company that once doubled year after year is now slowing down rapidly.  

On the plus side, SolarCity did report a profit of $4.6 million after allocating losses to non-controlling interests. Ultimately, that is a good sign for investors, even if growth slows in coming years.

Now what: This wasn't a bad quarterly report for SolarCity except for the growth numbers. Operating costs were lower than expected, installation costs were down, and the company was able to increase prices slightly. Those are good signs long term as SolarCity transitions to a more sustainable business model.

The transition from an ultra-high growth company to a more sustainable energy company is a tough one for SolarCity, and that's what investors are grappling with today. Falling growth plans and the failure to meet its own targets can cause growth investors to run for the exits, but that may not be a bad thing. SolarCity plans to be cash flow breakeven by the end of the year and has built a financing model that will allow it to build solar systems and finance them with no cash from SolarCity itself. That's a great place to be in the solar industry, even if growth figures are disappointing investors today.