Gross, now 49, is again building solar power projects, albeit after a lengthy detour through the early days of the Internet.

His company, Idealab, created a slew of Web businesses in the 1990s, including pay-per-click advertising pioneer GoTo.com and online toy retailer eToys, which he said eventully "outspent its leash."

"Everything we touched was turning to gold for a while, and then the crash came," Gross said in an interview at his headquarters in Pasadena, California. He is not related to the Bill Gross who manages bond fund Pimco.

In its heyday, Idealab was planning an initial public offering, had 5,000 employees and locations in Boston, San Francisco, Pasadena and London. That is down to between 500 and 1,000 employees and one office now.

In that period, he also faced accusations -- since dismissed or settled -- by some of Idealab's shareholders that he used company assets to finance a lavish lifestyle.

Two years ago, Gross was spared financial ruin again when Idealab shareholders agreed to pay off a US$50 million personal loan he owed to a bank.

As the Internet boom melted down, however, the California energy crisis of 2000 was rekindling Gross' teenage passion for alternative energy.

It turned out that solar technology had not advanced much since he gave up his mail order business to start writing software in the early 1980s.

"I thought, 'I could pick up where we left off.'"

Idealab dusted itself off and embarked on a mission few others were embracing at the time -- developing green energy technologies.

"The bust was the best thing that happened to Bill Gross," said technology forecaster Paul Saffo. "The lures of the bubble distracted him into doing things that were profitable, but didn't really matter."

A LASTING LEGACY?

A mechanical engineer by training, Gross is hands on. A smattering of solar projects are crammed onto the roof of the company's small brick building in downtown Pasadena, and Gross explains them all in rapid and minute detail.

His idea for small, pre-fabricated solar-powered thermal power plants, being developed by Idealab company eSolar, caught the eye of Oak Investment Partners and Google Inc.

Bandel Carano, a managing partner with Oak who also sits on eSolar's board, called Gross' solar thermal technology a breakthrough equivalent to the advantages of PC computing over mainframes.

Idealab is also behind Aptera Motors, the maker of a three-wheeled, space-age-looking car that Gross first imagined while he was sitting in traffic in Pasadena.

"I was stopped at a light thinking 'What a shame, I'm carrying all this metal along with me.'" He went home and started calculations for Aptera's first car, to be unveiled later this year with promised gasoline consumption of 300 miles per gallon.

Half of Idealab's 16 companies are focused on energy, and it has learned to be more disciplined than in the late 1990s.

"We used to have 40 ideas a year and do one company a month," he said. "Now we have 40 ideas a year and we're doing one company a year."

And this is no repeat of the Internet boom, when companies with little revenue and no profits were able to attract millions of dollars in capital, he said.

"Now we say 'Treat these dollars as if it's the last dollars you'll ever see.'" (Reporting by Nichola Groom; Editing by Eddie Evans)