Opposition to power lines from landowners and neighbors, local officials or environmental groups, especially in rural areas, makes expansion difficult  even when the money for it is available. And some experts argue that in the absence of a broader national effort to encourage cleaner fuels, even the smartest grid will do little to reduce consumption of fuels that contribute to climate change.

In fact, energy experts say that simply building a better grid is not enough, because that would make the cheap electricity that comes from burning coal available in more parts of the country. That could squeeze out generators that are more expensive but cleaner, like those running on natural gas. The solution is to put a price on emissions from dirtier fuels and incorporate that into the price of electricity, or find some other way to limit power generation from coal, these experts say.

The stimulus bill passed by the House includes $6.5 billion in credit to federal agencies for building power lines, presumably in remote areas where renewable energy sources are best placed, and $2 billion in loan guarantees to companies for power lines and renewable energy projects. The bill also includes $4.4 billion for the installation of smart meters  which, administration officials say, in combination with other investments in a smart grid, would cut energy use by 2 percent to 4 percent  and $100 million to train workers to maintain the grid.

About 527,000 miles of high-voltage transmission lines stretch across the United States, most installed many decades ago.

Everyone agrees that more lines are needed. But some industry experts argue that the problem of making the grid greener goes well beyond upgrading and expanding the existing power lines. The grid, they say, was set up primarily to draw energy from nearby plants and to provide a steady flow of electricity to customers. It was not intended to incorporate power from remote sources like solar panels and windmills, whose output fluctuates with weather conditions  variability that demands a far more flexible operation.