But they hadn’t funded the program. Federal budget rules require an appropriation to cover the risk that projects will fail and cost taxpayers money. How much would be needed was hard to determine, because there isn’t much of a track record for renewable technology or, in the past three decades, nuclear plants. In early 2009, as part of economic stimulus legislation, Congress provided the funds to cover expected losses.

Suddenly the Energy Department had a $33.6 billion grant program and a $38.5 billion loan-guarantee program, each bigger than the department’s normal annual budget, which was mostly devoted to the stewardship of the nation’s nuclear weapons stockpile.

The department initially had only nine people to review applications for loan guarantees. And there were lots of applications. The loan guarantees — and later direct loans from the Treasury’s Federal Financing Bank — drastically lowered the cost of financing a project. And finance costs, said Ted Meyer of First Solar, often equal the expense of purchasing all the modules in a solar project.

“It was a manna from heaven. Too good to be true,” said Michael Butler, chief executive of Cascadia Capital, an investment bank that has arranged financing for more than 20 clean-technology ventures. “Every proposal we saw planned on getting a government loan.”