WASHINGTON – An unapologetic Energy Secretary Steven Chu defended a half-billion-dollar federal loan to a solar-panel manufacturer that went belly up, even as he told a House committee Thursday that he was unaware of dozens of key details that led to the debacle over Solyndra Inc.

Under hours of hostile questioning from Republicans on the House Energy and Commerce Committee, Chu declined several opportunities to say he was sorry, but acknowledged that in hindsight the deal was “extremely unfortunate” and “regrettable.”

“Certainly knowing what I know now, we’d say ‘no,’” Chu told the energy panel’s subcommittee on investigations.

Rep. Cliff Stearns, R-Fla., chairman of the subcommittee, said after the hearing that Chu should be fired. “He has failed the test. The fact that he’s unaware of so many things makes me think that he’s not the best person for the position,” he said.

Chu insisted that politics played no role in his department’s decision to loan Solyndra $528 million before it went bankrupt and laid off 1,100 workers.

Testifying under oath on a widening controversy, Chu took responsibility for the disastrous 2009 loan, but said he was unaware of many details about the loan or financial problems that Solyndra faced — including predictions by Energy Department staff two years ago that the company was likely to face severe cash-flow problems.

Chu blamed the company’s demise on “totally unexpected” market changes — including an influx of cheap imports from China and the collapse of the European market for solar panels — that led to a steep decline in the price of Solyndra’s product.

Solyndra, of Fremont, Calif., was the first renewable-energy company to receive a loan guarantee under the 2009 stimulus law. Since then, the company’s implosion and other revelations have become an embarrassment for Chu and Obama and a rallying cry for GOP critics of the administration’s green energy program.