President Carter yesterday sent congressional energy committees details of a standby rationing plan that could limit autombile owners to as little as 2 gallons of gas or less a day if gasoline supplies are deemed in "severe shortage."

Department of Ehergy officials said that the president would not implement gasoline rationing by coupon unless there was a "severe shortage." They suggested that a 20 percent shortfall in oil supplies would be severe.

The current Iranian oil shortage has reduced U.S. imports by 5 percent.

Both the president and Energy Secretary James R. Schlesinger have repeatedly said that rationing would be used only "as a last resort." But it was, nonetheless, among the list of long-promised measures to deal with the Iranian oil squeeze that the White House sent to Capitol Hill yesterday.

Aside from rationing, the president. also proposed mandatory conservation measures including ending Sunday gasoline sales, banning decorative sighting and imposing lower thermostat settings. In aggregate they are intended to make up for lost Iranian oil deliveries to the United States.

A Department of Energy spokesman, James Bishop, said last night that there is no specific gallon-per-day rationing figure in the proposal.

Yesterday presidential press secretary jody Powell said that the rationing plan and conservation package had been sent "informally" to the congressional energy committees, and that the details of the plan would be made public after the administration had briefed the congressional leadership on its content.

For some weks Schlesinger has been telling Congress that the administration would deliver the plan by Feb. 26.

Either house of Congress can veto the president's conservation and rationing package within 60 days.

The Energy Department sent the proposed rationing and conservation plans to the White House Friday, touching off a round of consultations between the Office of Management and Budget and the president's domestic policy staff over the plans' details.

The White House, at one point ruled out the ban on decorative lighting, arguing that savings -- 4,000 barrels a day against nearly 2/ million barrels a day in oil consumption -- were marginal. Following a direct appeal by Schlesinger to the president on Saturday, however, this measure was restored to the package. Schlesinger said it was necessary as a symbol of energy conservation.

DDE officials say that banning weekend gasoline sales would save from 171,000 to 327,000 barrells of oil a day, with the most probable savings set at 246,000 barrels a day. Weekend or Sunday closings, however, are expected to be met by growing opposition in Congress from the tourism industry.

Mandatory thermostat settings, 80 degrees during the summder and 65 degrees in winter, are estimated to save from 336,000 barrels to 391,000 barrels a day, depending upon the season.

An earlier proposal to limit commuter parking was ruled out by DOE.

The administration's rationing plan would limit gasoline supplies on the basis of registered vehicles, rather than licensed drivers as offered in a rationing plan proposed by former president Ford in 1977. It would allow from 40 to 45 gallons per vehicle per month -- depending on the supply shortage.

DOE says it would cost up to $1.5 billion a year to implement, and that each rationing coupon would be worth five gallons when distributed.

These coupons, in turn, could also be sold on the "white market" by car owners for what DOE estimates would be about $1 a gallon. Under the DOE plan, commercial and private vehicles would have equal access to gasoline.

DOE has 4.8 billion unserial numbered coupons in storage in Colorado that could be used if the program were put in effect.

Deputy Energy Secretary John F. L'Leary has said that if the department goes to coupon rationing. DOE would prefer to have the coupons serialized before they are distributed.

Another potential problem with the existing coupons is that unless they are altered they could be used in vending machines that give change for a dollar.

Under the administration's plan. checks for each registered vehicle would be mailed to woners who would "cash" them for coupons at distribution points such as banks or post cooices. The coupons in turn would be required to purchase gasoline at a service station, with each coupon worth five gallons.