What: Shares of companies with exposure to residential solar are getting hit hard today with SolarCity Corp (NASDAQ:SCTY.DL) falling 12%, Vivint Solar Inc (NYSE:VSLR) down 15%, and Vivint's acquirer SunEdison Inc (OTC:SUNEQ) dropping 10%. TerraForm Power (NASDAQ:TERP), which will be buying residential solar assets as part of SunEdison's acquisition of Vivint Solar, also dropped as much as 11% today.

So what: A number of factors are converging to hurt companies with exposure to residential solar today. Oil prices dropping to $28 per barrel hasn't helped energy stocks, even though solar companies very rarely compete directly with oil.

The bigger problem is Nevada's recent ruling that eliminated net metering for homeowners and added fixed fees to customers' bills. Shares of SolarCity plunged on the ruling last week, but investors are clearly seeing more downside than they did when the ruling came out. 

Image source: SolarCity.

Vivint Solar doesn't have a presence in Nevada, so that particular ruling won't be bad for it, SunEdison, or TerraForm Power. But the fact that utilities are learning how to fight off residential solar is scary given the risks already associated with the complex merger. If utilities across the country start to add fixed fees or eat away at net metering in any way, it could lead to fewer opportunities for growth, which is something the residential solar industry doesn't need right now.

Now what: Nevada is an outlier when it comes to utilities and solar, always having an adversarial relationship with rooftop solar. And it's a very small percentage of the solar market at the moment.

But this adds more fuel to investors' questions about how much value they can count on from residential solar companies long-term. The assumptions companies like SolarCity and Vivint Solar use when calculating retained value assumes that customers will pay contracts as planned for 20 or 30 years with very few defaults. If you're projecting growth, it's easy to see how these companies have a long runway of growth given a low penetration rate of less than 1% of the country -- or at least that's how investors used to think about these stocks.

But if the rules of the game change, the value solar companies generate could change dramatically. Solar companies and customers are already suing Nevada regulators to change the rules put in place for 2016, but investors should realize by now they can't assume today's net metering rules will continue unchallenged forever.

No matter how you feel about utilities or rooftop solar, it's impossible to deny that the certainty of value creation from residential solar in both past installations and future installations is lower than it was just a month or two ago. The problem is we don't know how much the value of rooftop solar has changed, or if this is just a temporary setback. That uncertainty is hurting shares of solar companies today, but don't be surprised if the pain isn't yet over as companies adjust value creation numbers to adapt to the new competitive environment they face in the U.S.