One of the most sacred of perquisites on Capitol Hill, free parking, may be offered as a sacrifice to the energy crisis.

"It is clearly time to end this privilege," Sec. Charles H. Ppercy (R-Iii.) said yesterday during hearings on a bill that would eliminate 10,000 parking spaces now used without charge by employes of Congress and the Supreme Court.

When Percy offered a similar bill two years ago, it got only 25 votes on the Senate floor, and he was "booed by his own staff" for the idea.

But that was before the energy crisis, and before President Carter proposed to eliminate 30,000 free or cut-rate parking spots for executive branch employes.

Hill observers believe the pressure of the Carter edict and the raised consciousness resulting from the energy crisis make chances for passage this year much brighter.

"If you have a free lunch or a free ride, you don't want to give it up," Percy said, "but taxpayer-subsidized parking for government employes is one of those remaining traditions which symbolize our past cavalier attitude towards oil."

Mayor Marion Barry, who was the leadoff witness at the hearing before the Senate Governmental Affairs Committee, estimated that imposition of commercial parking rates at government garages and lots, would reduce the daily auto trips to the downtown area by 27,5000.

Such a cutback, Barry said, would save 15,000 gallons of gasoline a day, or 3.7 million gallons annually.

Barry said cut-rate parking creates "an economic incentive for people to drive to work."

The 40,000 free or reduced-rate parking spaces in the District and its suburbs amount to an $18 million annual subsidy to U.S. workers who get them, Barry said.

The major challenged the idea that giving preference in federal parking facilities to car pools is greatly reducing the number of autos. While federal officials claim the average number of riders in their car pools is about four persons, the mayor said, traffic counts taken during rush hours on major streets serving downtown show&only about 1.4 persons per car, and yet nearly half of the employes in this area work for the federal government."

The Percy bill would extend the reach of an order, announced April 5 by the president, that will end cutrate parking for executive branch employes by Oct. 1.

Thomas M. Downs, associate administrator of the Federal Highway Administration of the Department of Transportation, testified that nearly $1 million of the $18 million subsidy to federal workers here goes to DOT employes.

Assigned spaces in the DOT building downtown are rented to employes for $6.85 a month, Downs said, while commercial lots in the area charge as high as $80 a month.

Downs said his testimony in behalf of the Percy bill means the administration supports a law that would encompass the president's directive and federal employes outside the authority of the executive branch.

Both the president's order and the Percy bill would affect federal employes and officials nationwide. But aside from executive employes, only about 2,000 parking spaces are at stake for legislative and judicial branch employes outside the Washington area, most of them in small groups at federal courthouses, a DOT spokeswoman said.

Donald McIntyre, national vice president of the American Federation of Government Employees denied that free parking was a fringe benefit for federal employes, 700,000 of whom are represented by his union.

McIntyre said a random survey of large, private employers in the Washington area showed that many do not charge their employes for parking. None of the suburban employers contacted impose a charge, McIntyre said.

Charging the full rate would hurt handicapped and lower-paid federal workers the most, the union official said. McIntyre suggested that the question of parking charges be made part of the bargaining process with employe unions.

Other opposition was voiced by two Republican committee members, Sens. Charles McC. Mathias of Maryland, and Ted Stevens of Alaska.

Mathias dumped petitions signed by 10,000 federal employes on the desk in front of the acting chairman, Sen. David Pryor (D-Ark.), saying at petitioners do not work near a Metro stop and therefore do not have a convenient alternative to driving.

Stevens, who noted that he had been a federal employe for eight years before being elected to the Senate, called the proposal "just another effort by Congress to make government workers scapegoats for public policy."

Stevens said there already is "a serious morale problem among federal workers and no wonder." They have been subjected to a pay cap, hoping for a 5.5 percent pay raise when others get 7 percent; must worry about rumors of having their retirement benefits merged into Social Security, "and now this so-called tax on the chin again," Stevens said.

If the proposal is approved by Congress, it would phased in, with half-rate parking to the charged beginning in October 1980 and full commercial rates charged by October 1981.