A hipster in dark shades coos: "Use the sun, the wind and the rivers to power my computer?" And then she gives her verdict: "Cool."

It's an advertisement for an electric company in the new world of energy deregulation, where for the first time consumers are being offered a choice of what kind of electrons enter their homes -- those generated by traditional but polluting sources, such as coal burning or nuclear fission, or by renewable, cleaner "green" power, such as wind turbines, solar arrays and small dams.

This is a most radical experiment, and its advocates believe it could be the beginning of a massive, decades-long overhaul of the way electricity is generated across the nation.

Over the next few years, at least a dozen states will deregulate their electricity markets and allow competition. Consumers in Maryland and Texas soon will be offered a range of cleaner energy products.

But to see how the experiment is going, you have to visit Pennsylvania or California, the only states offering competitive green power to consumers.

Here, for the first time, the power to decide how electricity is produced is being transferred from monopolistic utility companies to a free market, where consumers can select sources that pump less pollutants into the air and reduce the load of carbon dioxide that many scientists believe might be responsible for global warming.

The idea is that instead of relying on government regulations, consumers and the market can work to clean up the environment.

Or not.

If consumers do not choose the greener options, which often cost more, or if state and federal governments stop offering subsidies that make renewable energy cost less, then some skeptics of this experiment fear that the fledgling green power market may die and reverse a trend of using more renewable sources.

"It's not that the green market is bad. It's good. It makes people feel better. But unless enough consumers choose the green sources of energy, this will have absolutely no appreciable impact on the environment," said Nancy Rader, an independent energy consultant who wrote an assessment of green electricity products for Public Citizen, a Washington consumer advocacy group.

In California, the market opened in April 1998, ending decades of monopolistic control by a handful of utility companies and municipal providers. Before, a consumer did not have much, if any, choice. There was one utility company per region. Now, there could be dozens.

There is not much difference in the price of a generic kilowatt hour of power, the measurement that appears on electric bills. So what these companies are offering is the option to switch from electricity generated by coal, natural gas or nuclear power plants -- the traditional sources -- to electrons produced by such sources as wind or sunlight.

The average customer in California, for example, receives electricity from a "power mix": about 35 percent natural gas, 24 percent from large dams, 17 percent coal, 14 percent nuclear and 11 percent from renewables.

But now consumers can purchase one of a dozen different energy mixes from marketers. By telephone or over the Internet, they can switch from the current provider to such products as GreenMountain.com's "Wind for the Future," which promises 100 percent renewable power, with 25 percent coming from newly constructed wind turbines.

Or, they can sign up with other marketers' products, such as Clean Choice 100, Earthsource 2000, Greensmart, Earth Choice 100 or Nature's Power 100.

What makes shopping for green power possible is that energy from generating stations miles away -- or in another state -- enters the "grid," the centralized distribution network. Then, in California, that electricity is bought and sold on the Power Exchange, which operates like a commodities trading pit, where each marketer bids and buys the next day's electricity based on need.

To buy green power, the wires or systems in a home or business do not need to be adjusted. The electrons flow through the system the same way, whether they are generated by the sun or a lump of coal. Also, because the power from all sources gets mixed up on the grid, a consumer who buys green does not get only power from renewables. They do support green electric generation, but the electrons that enter any one home could come from anywhere.

Energy marketers promote their green alternatives on billboards, in direct mailings to homes, and in newspaper and magazine advertisements.

One ad, hawking the green menu of new energy options, shows a Susie Homemaker type laying out the family dinner but wondering, "Okay, but what is biomass generation?" Another contains a pitch by soft rock singer Kenny Loggins.

Several municipal governments around the state have opted for green power, such as Santa Monica, San Diego and San Jose. So have some companies, such as the Los Angeles Dodgers baseball team.

All this happened because of energy deregulation, which is forcing electricity providers in California to compete and sell their services, much like the competition that broke out after the telephone business was deregulated. The motivation to deregulate was not to offer green power but because Californians paid some of the highest bills per kilowatt hour in the country.

So far, the new offerings are meeting with small success. In Pennsylvania, about 10 percent of customers have switched power companies, and many of them have chosen to buy their electrons from green marketers. In California, the nation's largest market, about 1 percent of customers -- or about 116,000 homes -- have switched to green.

"We think this is a good start, but that is what it is, just the start," said Marwan Masri, manager of the renewable energy program at the California Energy Commission.

One challenge, says Masri and others, is that most consumers do not yet know they have a choice because for decades no one thought of "shopping" for electricity. You paid your bills to the monopoly, and you had lights.

Kirk Brown, a director at the Center for Resource Solutions in San Francisco, whose program Green-e gives something akin to a Good Housekeeping seal of approval to renewable marketers, likens the early days of energy shopping to the first efforts to get Americans to recycle their trash. At first, recycling was a novelty, done only by the truly committed, Brown said. Now, recycling is ubiquitous.

There is not much difference between the cost of power from coal or natural gas. But renewable energy costs a penny or so more per kilowatt hour; green power is not quite competitive.

So far, society is making up the difference. Consumers in California get a subsidy for selecting green power, making the cost about the same as traditional electricity. But those subsidies are scheduled to end in 2001, and it is an open question whether consumers will be willing to pay more or whether costs for renewables will continue to drop.

One of the criticisms of green power is that most of the renewable power is not produced by new facilities but by already existing wind farms and solar arrays. Some of the marketers have pledged that for each new bundle of customers, they will commit to constructing new renewable power-generating stations. GreenMountain.com, for example, has built the largest solar array in Pennsylvania and is erecting new wind turbines in California, based on new customer demand. But the efforts are still small.

If millions of consumers choose green, it will support existing renewable stations and the construction of more. But no one knows whether green power will catch on.

There are some encouraging signs, though. In Los Angeles, the local municipal utility, the Department of Water and Power (DWP), which is a monopoly, is trying to improve its image because the power department may be deregulated in 2002.

"We are the big ugly power utility that hadn't gone green," said Angelina Galiteva, director of strategic planning at the DWP, which delivers power to residents in the city of Los Angeles, based on 70 percent coal and nuclear, making it the "dirtiest" utility in the state.

But the DWP is now offering its customers a green option and promises that for each bundle of new green consumers, it will support new wind or solar stations. The utility has signed up about 1.5 percent of its customers for green power in the past four months and hopes to have 10 percent by April.

"You're not doing much to clean up the air unless you reduce your dependence on coal and start using more wind, hydro or solar," she said.

Many environmentalists are divided over this experiment. They support green power but worry that if left to the free market, without government subsidies and policy support, renewables will not make much of a dent in pollution. Answers to such questions will come in about 10 years.