Shares of solar power companies enjoyed a huge bounce at the end of 2015 thanks to the lengthy five-year extension of a key solar tax credit in the United States and a major global accord to combat climate change that promises to spur development of renewable energy.

But a sharp slide in the price of oil to 12-year lows this week has rekindled long-held worries that low fossil fuel prices will sap demand for renewable energy sources. Many say those worries are misplaced, yet they have affected share prices.

In 2015, solar investors worried about dropping oil prices, an oversupply of new equity issues and concern that a U.S. interest rate hike by the Federal Reserve could curb the appeal of investing in bundles of solar assets with long-term utility contracts known as "yieldcos." Still, the MAC Global Solar Energy Stock index soared 17 percent in the last month of 2015, a strong ending to a volatile year.

But the euphoria did not last long. The index has dropped 16 percent so far in January, due in no small part to oil's 20 percent slide so far this year. Oil briefly fell below $30 a barrel on Tuesday for the first time in 12 years, and the solar index tumbled 2.75 percent.

Oil and solar do not directly compete with one another because oil is rarely used to generate power, and demand for solar is largely driven by government mandates and incentives.

"They are not substitutes," Raymond James research associate Angelica Jarvenpaa said of crude oil and solar energy. "However there is probably an impact on market psychology."

Some investment funds, Jarvenpaa said, have sold off their energy holdings altogether as the oil market carnage has intensified and caused financial pain to oil producers across the world. Solar and renewables, as energy stocks, are often dumped along with other energy shares even though solar installations are expected to log a 30 percent increase for 2015 and the cost of solar has come down so much that it remains cost competitive with traditional energy sources in many places.

Zevin Asset Management LLC, a Boston-based investment firm, has studied the correlation between oil prices and the value of solar stocks and found it typically lasts months, but not years.

"We are seeing a bit of a short-term correlation, but we don't think it affects the fundamentals of the companies and we continue to see strong demand for solar," said Amber Fairbanks, a portfolio manager at Zevin, which owns shares of U.S. solar companies SunPower Corp and First Solar Inc.