What: Shares of NRG Yield, Inc (NYSE:CWEN) plunged as much as 15% today after issuing a disappointing earnings report.

So what: Operating revenue increased from $173 million a year ago to $217 million but cash available for distribution fell from $43 million to $26 million. Since cash for distribution is the main goal of yieldcos that's a bad result for NRG Yield.  

Management also lowered full-year 2015 adjusted EBITDA guidance from $690 million to $660 million and lowered cash available for distribution guidance by $35 million to $160 million.

Now what: Historically low wind generation production was blamed for the miss, but it's concerning for investors looking for cash flow from the stock. Long-term, this is probably not an ongoing problem but it shows that renewable energy can have ups and downs like other energy resources. I would be cautious buying NRG Yield given the falling cash flows but if management can prove in coming quarters it was indeed a one-time event then I would feel comfortable buying in.