As U.S. demand for electricity steadily rises, producers face a choice: Cheap energy will not be the cleanest, and clean energy may not be the cheapest. At present the electricity sector contributes about 40 percent of U.S. emissions of carbon dioxide, which means nearly 10 percent of world emissions. Generating technology has become a political issue in which the central question is how much consumers are prepared to pay for progress in reducing emissions.

At present, half of the United States' electricity is generated with coal. It's the least expensive and most easily accessible of all fuels, but it also produces the most carbon dioxide -- and other pollutants -- per unit of heat.

The alternatives aren't simple. Over the past several decades, some utilities built power plants to run on natural gas, which produces much less carbon dioxide in relation to its heat content. Almost a fifth of U.S. power now comes from gas. But that swing is a major reason for the huge increase in gas prices over the past several years, and thus the power industry is moving back toward greater reliance on coal.

Another possibility is nuclear energy, which produces another fifth of the power supply. But the U.S. political system does not seem to be able to resolve the disputes over what to do with radioactive waste from reactors. And greater reliance on nuclear energy raises immense questions of security, because of the technology's close association with nuclear weapons.

Oil is no longer generally used for electric generation, partly because of environmental reasons but mostly because of the instability of its price. As for hydroelectricity, all of the most promising sites for dams are already being exploited.

Renewable energy provided, in 2004, 2.3 percent of the country's electricity, a proportion that had not changed much of the previous decade. About half of the state governments have enacted legislation to push power plants toward greater use of renewables. While progress will be steady, it is unlikely to be rapid.

Federal policy is centered on support for research and development to reduce the pollution that coal causes. Congress has for many years funded clean coal projects. In early 2003, President Bush announced a project called FutureGen to build, over the next decade, a pilot plant to produce both electricity and hydrogen while sequestering the resulting carbon dioxide and other pollutants. The president promised that it would be the cleanest power plant in the world. But until it is much farther along, it will be difficult to know how much its power will cost.

Electricity restructuring, while currently stalled in the United States, could result in substantial changes in the mix of generation technologies employed to produce electricity, in the efficiency of power plant operations, and in the price and quantity of electricity traded in the marketplace.

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