These are tricky days for the nation's electric utilities.

Demand is uncertain. Supply is problematical. Big customers, once a captive audience, more and more frequently have options to shop for the best rates.The resulting scramble to supply cheap and reliable electric power has led the federal government to re-examine its regulations.

"It has become clear that our current form of regulation is ill-equipped to deal with some of the uncertainties and risks associated with today's electric power business," said Martha Hesse, chairman of the Federal Energy Regulatory Commission.

Once, steady utility growth seemed as sure as snow in Siberia. In the 1960s, national consumption of electricity was increasing 7 to 10 percent a year.

Many utilities launched ambitious building projects to increase capacity, figuring they would easily recover the costs.

But runaway oil prices in the 1970s changed all that. Energy consumption decreased. Rates, which had dropped steadily for decades, began to rise.

Meanwhile, huge investments in nuclear generators began to sour. State commissions, pressured by consumer groups, began to refuse to pass along costs to ratepayers - and some big industrial customers began generating their own energy.

The government got into the act, too: President Jimmy Carter succeeded in pushing through Congress a 1978 law intended to encourage the development of small renewable energy and conservation projects.

The Public Utility Regulatory Policies Act required utilities to buy electricity from small solar, wind or water projects.

The utilities also had to buy power from industrial plants that could cogenerate electricity as a byproduct of the manufacturing process.

"When the law was passed, people had this image of two hippies up in the mountains grinding granola with a water wheel and producing electricity," said Tony Osbon, a financial analyst for Regulatory Research Associates in Washington. "But now these hippies are Fortune 500 companies."

As a result of the 1978 law, more than 10,000 megawatts, the equivalent of 10 large power plants, have been added to the nation's electricity capacity by these new kinds of generators.

Today, this comprises 7 percent of the total electricity capacity in the country.

"Nonutility power has grown into a multibillion-dollar business that is providing a majority of the new generating capacity in several large regions of the country," said Mrs. Hesse, the FERC chairman.

The FERC likes what it sees and wants to encourage more competition in the business of generating electricity.

New proposals on the table would allow private companies to bid to sell power to the utilities, perhaps in competition with the utilities' own plants.

Several states already have competitive bidding. In Maine, the state's Public Service Commission sets a ceiling price for competitive bids, then the utility picks the most attractive bids under that price.

Since 1983 the Maine commission has gradually lowered the ceiling price from 9 cents per kilowatt hour to 5.5 cents.

Virginia Power is preparing to sign contracts for 2,041 megawatts of power generated by independent entrepreneurs.