Reacting angrily to reports that county residents are buying gasoline storage tanks in order to beat the automobile energy crunch, the Fairfax County Board of Supervisors unanimously enacted on emergency ordinance yesterday banning further installation of the tanks.

"I feel that an emergency exists," said Supervisor Martha V. Pennino (D.-Centreville), who made the request that was unanimously supported by the nine-member board. "This is hoarding. I just can't believe that some people would do this."

The Washington Post reported Sunday that scores of well-to-do people in the Washington suburbs have recently installed gasoline and diesel fuel tanks at a cost of $300 to $3,000 at their homes. Among those who have purchased the tanks is former Navy secretary J. William Middendorf, who last month had a 4,000-gallon gasoline tank installed in the front yard of his four-acre estate in McLean. The tank is large enough to supply the average American car with gasoline for more than seven years of operation.

Pennino said yesterday that Middendorf, as a former public official, had shown the "worst form" of leadership for a country trying to cope with a shortage of gasoline.

"Middendorf says that he has to travel. Well, so do a lot of people. A carpenter in Fairfax County has to travel to his job. Working people here have to travel as much as some of our celebrities," Pennino said.

Middendorf, who was appointed secretary of the Navy in 1974 by then President Nixon and who now is president of Financial General Bankshares Inc., said yesterday that he has not yet filled his underground tank with gasoline and that he has no intention of doing so until the gasoline shortage is over.

Last week Middendorf said he bought the tank, which cost $3,000 installed and is one of the largest gasoline tanks located at a private home in the Washington area, because he wanted "an insurance policy for the future."

"I find myself in a situation where I have to get places," he said. "I'm in constant demand from a business, social and political point of view."

Supervisor Warren I. Cikins (D-Mount Vernon) another member of the Fairfax board who expressed outrage at private "hoarding" of gasoline, said that wealthy people should not be allowed to store gasoline in ways that other people cannot afford.

"We are all pretty much in his energy crisis together," Cikins said. "We should not allow individuals to say, 'I'll get mine first.'""

The county has the authority, under its zoning laws, to ban the installation of new gasoline tanks at private homes and to limit the amount of gasoline that can be stored there, according to zoning administrator Philip G. Yates.

The ordinance approved yesterday bans new tank installations at all residences except farms. It took effect immediately and will remain in effect for up to 60 days, during which time the supervisors will hold a public hearing and decide whether to adopt the ordinance permanenetly.

Prince George's County is the only major jurisdiction in the Washington area with a complete ban on gasoline tanks at private homes. In Montgomery County, about 100 permits for the tanks have been issued since the first of the year.

Officials say that consumer inquiries about where to buy and how to install the tanks increased at the same time Washingtonians began waiting in lines for gasoline.

Gasoline distributors in the Washington area and officials of the federal Department of Energy said it is virtually impossible to find anyone willing to deliver fuel to a private home.

Five oil distributors contacted by The Washington Post in Maryland and Virginia said they cannot guarantee gasoline to ppeople who want to install tanks. Some distributors advise prospective tank buyers not to waste their money.

DOE officials strongly recommend against hoarding gasoline or diesel fuel in home tanks. "All this will do is aggravate the internaional situation by reducing the already tight supply," said DOE spokesman Tom Tatum.