Renewable energy, such as from photovoltaic electricity and ethanol, today supplies less than 7 percent of U.S. consumption. If we leave aside hydroelectric power, it is under 4.5 percent. Globally, renewables provide only about 3.5 percent of electricity and even less of transportation fuels.

But increasing that fraction for the U.S.—as seems necessary for managing greenhouse gases, trade deficits and dependence on foreign suppliers—has at least three tricky components. The obvious one is how to capture the energy of wind, sun and crops economically. After that, the energy has to be moved from where it is easily gathered, such as the sunny American Southwest or the windy High Plains, to the places it can be used. And the third is to convert it into convenient forms. Most prominently in the last category, electricity for transportation has to be loaded into cars and trucks, either through batteries or perhaps as hydrogen.

In some ways, the field is galloping ahead. A recent study sponsored by the United Nations found that global investment in renewable energy in 2007 was $148.4 billion, up 60 percent from 2006. But new wind turbines and solar cells are joining an infrastructure with coal-fired power plants that seem to run more hours every year and that are multiplying as well.

And although solar energy and especially wind have declined steeply in price over the past few years, they are competitive only when given subsidies or mandates. U.S. residential customers pay an average of 11 cents per kilowatt-hour (kWh) for power from a mix of coal, natural gas, nuclear and hydroelectric sources, but renewables are far pricier. Of course, all forms of energy get a carrot-and-stick treatment from governments, whether to provide work for coal miners or to prove that splitting the atom is useful for something besides bombs. But in many places, renewables get something even better: quotas. And rising prices for traditional fuels could help, raising the market to reach the renewables’ costs.

A carbon charge would also help; each $10 tax on a ton of carbon dioxide emitted would raise the price of a kilowatt-hour from a coal-fired power plant by about a penny. But the scale of the transformation is immense; in energy content, coal production is about 70 times larger than wind-energy production. The numbers for oil and natural gas are similarly daunting.

View a slide show overview of the elements of an energy system with a large contribution from renewables and the ways they might fit together.

Note: This article was originally printed with the title, "The Power of Renewables".