The United States has a rule in place to impose a 2.5 percent tariff on imported solar panels. Except, few knew about it until recent weeks.

The U.S. customs decided in January this year that the tariff exists, after an American Subsidiary of a Spanish company asked about it, reported the New York Times.

But that decision apparently didn't become widely known even though the customs service posted it on its website. When it became more widely known in recent weeks, bigwigs in the solar industry tried to keep it mum while meeting to figure out what to do, the Times reported.

One legal strategy is to challenge the custom officials' decision to classify solar panels as electrical generators. They issued the decision after determining that solar panels have become more complex electronic equipment (see ruling).

The tariff could cost importers about $70 million if they have to pay a double fine for failing to pay the money since January this year, according to the Solar Energy Industries Association.

The tariff would have the biggest impact on importers of Chinese solar panels, and those importers would include the American subsidiaries of Chinese solar panel makers such as Suntech Power.

China, along with Taiwan, accounted for about 28 percent of the world's solar panel production capacity in 2008, according to GTM Research. Europe had 30 percent while Japan had 17 percent. The United States took up 7 percent.

Europe remains the largest market for Chinese companies, though the United States is the new frontier to conquer.

Suntech, China's largest solar panel maker, saw its sales in the United States from 3.4 percent of its revenue in 2006 to 7.4 percent in 2008, according to its filing with the U.S. Securities and Exchange Commission. Europe's share was 77.7 percent in 2008.