A highflying Hong Kong stock raises concerns Getty ImagesA man and his child wear masks in Shanghai in December 2013 amid heavy smog.One popular way to bet on solar stocks, the Guggenheim Solar ETF TAN, -2.95% is up 11% this week, helped by factors that include a widely viewed documentary about smog in China. The solar exchange-traded fund has gained 32.3% in the year to date as of Thursday’s close, making it the best-performing U.S. ETF so far in 2015. That’s according to XTF.com data that excludes leveraged ETFs. The adjacent chart shows the Guggenheim Solar ETF’s rally. Two rivals aren't far behind, as both the Market Vectors Solar Energy ETF KWT, -2.83% and iShares Global Clean Energy ETFICLN, -3.15% boast year-to-date advances of 24.8%. Below are five reasons that might explain why solar is shining. 1. Crashing crude’s overhang abates: Solar stocks tumbled in the last three months of 2014, falling as crude-oil prices cratered CLJ5, +0.10% and investors bet that would mean less demand for alternative energy. But the solar industry’s latest quarterly earnings have showed that crude’s crash hasn’t hurt that much, according to Baird analyst Ben Kallo. “That’s one of the major drivers—the decoupling with oil,” Kallo told MarketWatch on Thursday. Oil prices also have managed to stabilize lately. Kallo said that if crude prices plunge again, solar stocks could get hit once more. 2. The yieldco effect: Investors have cheered the arrival of a new financing approach called a “yieldco.” It’s a strategy that solar companies can use to get projects off their balance sheets, but still maintain some ownership, Kallo said. 3. Chinese film, other buzz: A new Chinese documentary, “Under the Dome,” has been viewed online more than 200 million times, and the movie has “taken China by storm,” as NPR puts it. The Wall Street Journal noted that government censors eventually curbed discussion about the film, which has been likened to the 1962 book “Silent Spring” that helped spark the U.S. environmental movement. Even so, Jefferies analysts have said this could be a “watershed moment” for Chinese environmental policy, and investors ought to bet on industries such as solar and wind. (Hat tip to the FT) 4. A key stock soars in Hong Kong, but stirs concerns: Highflying Hanergy Thin Film Power Group 0566, -1.47% has become the biggest component of the Guggenheim Solar ETF, the Market Vectors Solar Energy ETF and the iShares Global Clean Energy ETF, and so it’s a big reason why those ETFs are rallying. Hanergy’s Hong Kong-listed shares are up 481% over the past 12 months, according to FactSet data as of intraday Friday. But Hanergy’s accounting has raised eyebrows, as a Barron’s blog post citing a Financial Times report says. These concerns underscore how investors always should check under the hood with every ETF to make sure they know what it’s holding. 5. Optimism over solar panels on U.S. rooftops: One of the best-performing segments of the solar industry is the U.S. rooftop market, Kallo said. Investors have been focusing on the “high growth” in that area, he said. Beyond the five potential drivers above, it’s worth noting the Guggenheim Solar ETF remains far below its 2008 peak, even if it’s on an upswing so far this year. See the chart below. The fund has $387 million in assets, according to ETF.com data. Victor Reklaitis