It’s not terribly often that a Republican investment banker becomes an advocate for the environment. In the case of Pacific Gas & Electric Co. (NYSE: PGC) CEO Peter Darbee, no one was more surprised by the transformation than the man himself.

After stints as an investment banker with Goldman Sachs (NYSE: GS), Citibank and Solomon Brothers and as CFO for telecommunications utility Pacific Bell, Darbee joined PG&E in 1999 as senior vice president and CFO. Five years ago, he was focused not on climate change and renewable energy, but on guiding the utility through bankruptcy proceedings. Since taking the helm in 2005, Darbee has had to think beyond the balance sheet and has shocked environmentalists and analysts alike by rising to the occasion and making PG&E not only profitable—with an average 7 percent year-on-year growth in earnings—but also an environmental leader with substantial investments in renewable energy sources, energy-efficiency initiatives and green-collar job training.

Although it may seem strange to some to see a numbers guy suddenly become a big-picture man, Darbee has credited his background in finance with putting him on the path to sustainability. Darbee declined to be interviewed for this article, but the story of his transformation is the stuff of legend. As CEO he famously brought in experts on both sides of the global warming debate and asked them to provide evidence to back up their opinions. Unimpressed by any evidence that the Earth wasn’t warming, Darbee concluded that global warming was real and methodically set about doing something about it. Step one for Darbee was, and continues to be, efficiency. “Some people talk about efficiency as the ‘fifth fuel,’ but we think of it as the first fuel,” he has said.

As a testament, good or bad, to his facts and figures mindset, Darbee has also always included nuclear energy as an essential part of the solution to the energy crisis, alongside solar, wind and geothermal power. Although some continue to criticize him for his financial and public support of nuclear energy, a handful of environmental groups and energy industry experts have begun to tout nuclear energy as a clean and renewable source.

In addition to specific PG&E initiatives, Darbee has become a vocal advocate of renewable energy on the world stage, and has thrown his weight behind several state and federal initiatives to mandate energy efficiency and introduce a cap on carbon emissions. In early 2008, as Darbee addressed the United Nations, he urged global leaders to stop making excuses for not addressing the energy crisis and start thinking about long-term national targets and timetables for reducing greenhouse gases.

“The first thing to remember is that climate is a long-term challenge,” he told the attendees. “If we’re nearsighted in our thinking, we’re going to make a lot of bad choices—both financially and environmentally—because we’ll misunderstand the risks and opportunities—or miss them altogether.”

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