Sunset Ford's 13 acres of glistening automobiles used to gleam like a Hollywood movie premiere under arclights at midnight, a publicity and security device that helped make brightly lit car lots a symbol of Southern California.

In the last few months, however Sunset Ford, with the encouragement of its local electric utility, has gone pitch dark at closing time. It has cut its use of electricity by 38 percent and installed radio-controlled switches on its showroom air conditioners, all symbolizing an unprecedented enthusiasm for conservation that is spreading to utilities and their customers all over the country.

Some West Coast utilities estimate that conservation has saved from 5 to 10 percent of their total electrical output. Others say they have been able to cancel or postpone plans to build new power plants because they were no longer considered essential -- because of conservation.

West Coast utilities, hardest hit by oil price increases and new constraints on nuclear power, have been the quickest to embrace new ways of saving energy. Portland General Electric Co. in Oregon has insulated nearly 16,000 electrically heated homes in the last three years, charging the owners nothing until they eventually sell the houses.

Pacific Gas and Electric Co. in Northern California has arranged low interest insulation loans for more than 23,000 homes. Southern California Edison has adjusted pumps on 100,000 swimming pools to reduce their drain on the system during peak day-light hours, a crucial matter in the conservation movement.

Eastern uitilities have also begun to experiment. The New England Electric System has produced a plan to reduce its use of foreign oil from 73 percent to 10 percent in 15 years. The company proposes to conserve energy through such measures as remote control, switch-offs on some consumers, burning trash to generate electricity and distributing 500,000 steel discs to cut hot water use in showers in half.

The Tennessee Valley Authority has eagerly offered no-interest loans for home insulation to its customers. The Dallas Power and Light Co. began last month to suffer large electric bill credit to customers who replaced broken down air conditioners with more efficient models.

The New England system found the decline in demand so great it decided it could scrap plans to build a huge nuclear plant at Charlestown, R.I., where they had been stymied in acquiring land anyway.

Pacific Gas and Electric ceased attempts to reopen a small nuclear plant near Humboldt, Calif., when it saw how successful the conservation effort seemed to be. The company also delayed by five years plans for a huge coal-fired plant in Solano County that would have provided enough energy to serve a city of 1.5 million people.

Although it is difficult to estimate how much conservation measures have saved nationwide, companies with the most aggressive programs put the amount at about 5 to 10 percent of their total output.

Pacific Gas and Electric Co. said it utilized the equivalent of 14.6 million barrels of oil in what would have been wasted energy in 1980, enough to provide 49,000 homes with electricity for 10 years.

Well-organized environmental groups on the West Coast have something to do with the surge here towards conservation, but several utility executives and economists say the principal motive is money. While America's consumers have been turning off lights and adjusting thermostats because of rising electric and fuel bills, utilities have seen the potential profits from building new power plants evaporate because of high interest rates and new environmental restrictions.

Conserving power, in many areas, has become cheaper than building new plants.

"The utilities look at it as dollars and cents," said Paul Greiner, a vice president of the Edison Electric Institute which represents investor-owned electric utilities. "If they can see pay-back in one or two years, they'll do it."

Higher prices to consumers, said University of Wisconsin's economist Charles Cicchetti, have caused per capita use of natural gas to decline in recent years and per capita use of electric power to hold steady.

But in many areas, particularly California and the Pacific Northwest, Cicchetti said, "the utility cost is more than the price they can charge under regulations, so they are losing money on marginal sales, and conservation makes a lot of sense."

Some utility executives say the change has influenced what kinds of people seek jobs with them. Few top engineering graduates interested in construction work seek out utilities, because utilities can no longer afford to build new power plants. But applications from college and university graduates with interest and some training in conservation planning have multiplied rapidly. "We got 500 applications for one position," said Jim Mitchell, a spokesman for Southern California Edison.

Contractors are also adjusting rapidly to the changes. Price have gone up for insulation in the Palm Springs area, where Southern California Edison has begun to offer non-interest loans.

The Tennessee Valley Authority had to alter its rules for no-interest insulation loans after home builders began to sell customers new houses at a discount without insulation and suggest new owners seek a TVA loan to finish the job.

In the Washington area, utilities say they have not been hit by an energy crunch quite as hard as on the West Coast and have not found the potential savings from conservation great enough yet to justify expenses such as subsidized loans for insulation.

The Virginia Electric and Power Co., however, has already reduced its projected peak demand for 1987 by 3,600 megawatts -- the equivalent of four large generating plants that would have cost $7 billion.

Everard Munsey, a former Arlington County Board member now working for Vepco, said the company was encouraging conservation through higher rates in summertime, free inspections of homes to determine where insulation is necessary, guidance to builders on producing more energy-efficient housing and a program to encourage some industrial customers to generate their own power through use of steam and combustible by-products that are now wasted.

The company is also experimenting with remote-control water heaters, which could be turned off briefly during peak demand daylight hours by a radio signal or a signal sent over the electric power line itself.

Marcia Schnedler, a spokesman for Potomac Electric Power Co., said the utility does not plan to build a new power plant for at least 10 years. Pepco also is experimenting with radio-controlled water heaters and air conditioners and has begun to charge its largest customers a premium for power used in peak hours.

Cutting demand at peak hours reduces the need to build new plants at high interest rates, a principal stimulus for many of the utilities' conservation efforts.

"We're not as innovative as the West Coast utilities," Schnedler said, "but we are in a different position then they are." Pepco grew rapidly in the 1950s and 1960s; the growth in customers has slowed now. The company also initiated several conservation measures during the 1974 oil crunch that appear to have reduced the necessity for more conservation now