When the builders of the Maple Ridge Wind Farm spent $320 million to erect nearly 200 windmills in upstate New York, the idea was to get paid for producing electricity. But at times, regional electric lines have been so congested that Maple Ridge has been forced to shut down even with a brisk wind blowing.

That is a symptom of a broad national problem. Expansive dreams about renewable energy, like Al Gore's hope of replacing all fossil fuels in a decade, are bumping up against the reality of a power grid that cannot handle the new demands.

The dirty secret of clean energy is that while generating it is getting easier, moving it to market is not.

The grid today, according to experts, is a system conceived 100 years ago to let utilities prop each other up, reducing blackouts and sharing power across small regions. It resembles a network of streets, avenues and country roads.

"We need an interstate transmission superhighway system," said Sudeen Kelley, a member of the Federal Energy Regulatory Commission, which governs many interstate transmission projects.

While the United States now gets less than 1 percent of its electricity from wind turbines, many experts are starting to believe that figure could hit 20 percent.

Achieving that would require moving large amounts of power over long distances, from the windy, lightly populated plains in the middle of the country to the coasts where many people live. Builders are also contemplating immense solar-power stations in the nation's deserts that would pose the same transmission problems.

The grid's limitations are putting a damper on such projects already. Gabriel Alonso, chief development officer of Horizon Wind, the company that operates Maple Ridge, noted that in parts of Wyoming, a turbine could make 50 percent more electricity than an identical model installed in New York or Texas.

"The windiest sites have not been built, because there is no way to move that electricity from there to the load centers," he said.

The basic problem is that many transmission lines, and the connections among them, are simply too small for the amount of power companies would like to squeeze through them. The difficulty is most acute for long-distance transmission, but shows up at times even over distances of a few hundred miles.

The transmission lines carrying power away from the Maple Ridge farm, atop Tug Hill near Lowville, New York, have sometimes become so congested that the company's only choice was to shut down - or pay fees to the grid operator for the privilege of continuing to pump power into the lines.