What happened

Shares of natural gas-based fuel specialist Clean Energy Fuels (NASDAQ:CLNE) rose 21.1% in April, according to data provided by S&P Global Market Intelligence. The company's performance outpaced the overall market -- the S&P 500 only rose 12.7% for the month.

However, shares of Clean Energy underperformed in March. Year to date, the company's stock is slightly underperforming the S&P 500, down 10.3%, compared to the index's 9.3% loss. 

A truck at a Clean Energy Fuels' filling station.

Image source: Clean Energy Fuels.

So what

Renewable energy stocks tend to underperform in periods of cheap fossil fuel prices. While Clean Energy's natural gas fuel isn't "renewable," it burns cleaner than petroleum-based fuels -- hence, the company's name. But at least renewables can argue that they're... well, renewable, which gives customers an incentive to buy even when the economics don't make sense. 

For Clean Energy Fuels, which operates more than 550 natural gas filling stations across the U.S., it's the same story when it comes to oil prices. In March, shares tumbled on fears that falling oil prices would make natural gas-powered vehicles less competitive with traditional gasoline-fueled options. That, in turn, would negatively impact the company's natural gas-powered filling stations.

With many U.S. oil and gas producers cutting their output due to the low price of oil, there are concerns that the U.S. supply of natural gas will shrink, as well. But because the U.S. doesn't import natural gas from overseas, that would likely cause natural gas prices to rise faster than oil prices, essentially forcing Clean Energy to take a double hit as natural gas fuel gets more expensive at the same time oil gets cheaper. These concerns caused Clean Energy's stock to lag the market for most of the month. 

It was only at the very end of April that crude oil prices began to rise significantly, and Clean Energy's shares rose right along with them.

Now what

Clean Energy Fuels has been a long-term loser. Revenue has slowly but steadily declined over the last five years, and the company has struggled to stay profitable. It's managed to pay down a significant portion of its debt during that time but more than doubled the number of outstanding shares in the process. Small wonder that Clean Energy's share price is down more than 50% since 2015.

With oil prices likely to remain low for the foreseeable future, Clean Energy Fuels is unlikely to suddenly turn itself around. Energy investors should probably steer clear.