The figures are listed in an online sales document issued last month on a trust investment product sold through China Minsheng Trust.

"I'm not sure about the revenue and profit gaps," said CLSA head of sustainable research Charles Yonts, who has written two reports on Hanergy TFP and its huge connected transactions with its parent since December.

The transactions committed Hanergy TFP to buy US$8.5 billion worth of production lines for making solar panels from the parent, which in turn will sell up to US$2.2 billion of panels to Hanergy TFP for building solar farms.

The lucrative deal has boosted Hanergy TFP's profit and shares in the past few years.

Hanergy has not responded to the 's request for information on what business made up the major differences between the revenue and profit figures of Hanergy and the Jinanqiao project in Yunnan province.

The lack of clarity about the assets and profit sources of privately held Hanergy would not help it build investor confidence in its listed unit, Hanergy TFP, whose multibillion-dollar transactions with its parent and the dearth of solar farms built by third-party customers using its panels have raised questions about their sustainability.

"I don't know how much of its parent's solar panel production capacity really exists and is in operation. You don't see much of its products in the field," said Frank Haugwitz, the founder of consulting firm Asia Europe Clean Energy (Solar) Advisory.

The parent's need to borrow billions of yuan in recent years from lenders through various trust investment products, which are less regulated compared to bank loans and carry much higher interest rates, has also raised eyebrows.

The group even borrowed 330 million yuan for six years by selling and leasing back one of chairman Li Hejun's private jets to Hong Kong-listed Noble Century Investment Holdings last month, according to Noble's filing to the stock exchange.

Hanergy TFP's rocketing share price and high valuation have raised concerns that it may be a bubble waiting to burst.

"This stock is strictly for speculation. There is little in its fundamentals you can rely on," GEO Securities chief executive Francis Lun Sheung-nim told the .

If the bubble bursts, it would hurt mainland investors, who have been buying the stock in past months through the Shanghai-Hong Kong Stock Connect that allows cross-border trading between the two cities.

Hanergy TFP ranked as the most actively traded stock in southbound trading in the scheme in 15 of the 21 trading days to March 25 and is among the top three for all but one of the 21 days.

The stock has jumped 240 per cent in the past four months, prompting market watchers including corporate governance activist David Webb to call it a bubble.

It is worth six times the value of US-based First Solar, widely considered the global leader in the thin-film solar business. It is also worth more than all of its industry peers in China combined. Most of its rivals saw substantial share price declines in the past six months.

Hanergy TFP's rally meant it has a market value of HK$275 billion, ranking it the 19th-largest Hong Kong-listed stock on Thursday, ahead of Hang Seng Bank and Sands China. Yet the company is scantily covered by analysts.

In December, Yonts said the listed unit was overvalued by at least 100 per cent, even assuming it could deliver on all its ambitious business plans.

Its share price has soared 140 per cent since.

While investors struggled to understand Hanergy TFP, they know even less about its parent.

According to the online sales document, Hanergy has issued an 18-month investment product through China Minsheng Trust to raise 600 million yuan. A form of non-bank borrowing, it is backed by the pledging of shares of the Jinanqiao project with the trust firm, which administers the scheme on investors' behalf.

Investors putting at least one million yuan in the product are entitled to receive a 10 per cent annual interest, much higher than bank deposits of 2 to 3 per cent.

Through China Credit Trust, Hanergy raised about 2.1 billion yuan through a three-year trust investment product, also backed by the project. Matured in September 2013, it paid investors an interest of 8 per cent.

Another two-year product through China Fortune International Trust raised at least 522 million yuan in 2010.

According to Hanergy's website, it has hydropower projects with a total generating capacity of more than 6,000 megawatts (MW), enough to meet power demand from more than three million mainland households.

Its website lists four operating hydro plants, including the mainstay 2,400 MW Jinanqiao plant, whose four generating units came on stream between 2011 and 2012. The other three, in Guangdong and Yunnan, have a combined generating capacity of 82 MW.

Hanergy has an 80 per cent stake in Jinanqiao.

The project is the world's largest hydro plant built by a privately owned firm, according to Sichuan Trust, which also issued a trust product for Hanergy.

Hanergy recorded revenue of 6.92 billion yuan and net profit of 790 million yuan in 2012, with total assets of 43.1 billion yuan and net assets of 5.7 billion yuan at the end of that year, according to Sichuan Huasheng, a company hired by Sichuan Trust to value Hanergy's assets.

Hanergy's non-hydro revenues appear modest, based on its website's description of its assets. They include 131 MW of operating wind farms and 3,000 MW of solar-panel manufacturing plants, whose utilisation has been low.

According to a January report by the , filings by eight of Hanergy's nine solar-panel factories to the State Administration for Industry and Commerce showed combined sales of only 315 million yuan and a combined loss of 38 million yuan in 2012.

It quoted Frank Dai Mingfang, a director of Beijing-based Hanergy and the chief executive of listed Hanergy TFP, as saying that the performance of the factories in 2013 "should be much better and 2014 should be good", without giving figures.