By Andrew C. Revkin and Matthew L. Wald
    The New York Times

    Monday 16 July 2007

    The trade association for the nuclear power industry recently asked 1,000 Americans what energy source they thought would be used most for generating electricity in 15 years. The top choice? Not nuclear plants, or coal or natural gas. The winner was the sun, cited by 27 percent of those polled.

    It is no wonder solar power has captured the public imagination. Panels that convert sunlight to electricity are winning supporters around the world - from Europe, where gleaming arrays cloak skyscrapers and farmers' fields, to Wall Street, where stock offerings for panel makers have had a great ride, to California, where Gov. Arnold Schwarzenegger's "Million Solar Roofs" initiative is promoted as building a homegrown industry and fighting global warming.

    But for all the enthusiasm about harvesting sunlight, some of the most ardent experts and investors say that moving this energy source from niche to mainstream - last year it provided less than 0.01 percent of the country's electricity supply - is unlikely without significant technological breakthroughs. And given the current scale of research in private and government laboratories, that is not expected to happen anytime soon.

    Even a quarter century from now, says the Energy Department official in charge of renewable energy, solar power might account for, at best, 2 or 3 percent of the grid electricity in the United States.

    In the meantime, coal-burning power plants, the main source of smokestack emissions linked to global warming, are being built around the world at a rate of more than one a week.

    Propelled by government incentives in Germany and Japan, as well as a growing number of American states, sales of solar panels made of silicon that convert sunlight directly into electricity, known as photovoltaic cells, have taken off, lowering manufacturing costs and leading to product refinements.

    But Vinod Khosla, a prominent Silicon Valley entrepreneur who focuses on energy, said the market-driven improvements were not happening fast enough to put solar technology beyond much more than a boutique investment.

    "Most of the environmental stuff out there now is toys compared to the scale we need to really solve the planet's problems," Mr. Khosla said.