What: Shares of SunPower Corporation (NASDAQ:SPWR) dropped 17% in September despite a booming solar market in the U.S. and abroad. The question is: what's going on with solar stocks right now?

So what: Solar stocks have followed a herd mentality over the past year, and last month's sell-off was no different for SunPower. The company's yieldco, 8point3 Energy Partners, dropped as other energy yield vehicles sold off, despite the fact that it has long-term contracts for the power it makes.

SPWR Chart

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The market is currently afraid that all energy sources are getting cheaper, which will make solar less attractive and sink SunPower's profits.

Now what: Long-term, there wasn't any change in SunPower's prospects -- 8point3 Energy Partners has enough cash to buy the projects it's committed to through the middle of next year, and oil isn't a big competitor to solar in the first place. In fact, analysts have increased SunPower's expected earnings for this year from $1.04 per share to $1.57 per share over the past 90 days. That puts the stock at a P/E ratio of 14 in a growing market. That's a great price for an industry leader as long as you have a long-term horizon.

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