President Carter plans to announce his new energy plan on March 29, congressional leaders said yesterday, but key decisions on decontrolling oil prices and conserving fuel still have not been made.

Members of Congress who participated in a leadership brakfast with Carter at the White House quoted the president as saying he still has not made up his mind on many details of the new plan.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said Carter intends to consult further with key lawmakers before delivering his energy speech to the nation.

Meanwhile, a congressional staff study suggested that a presidential decision to lift price controls from crude oil could cost consumers from $5 billion to $14 billion a year in higher fuel costs.

The oil price study, by the staff of the House energy and power subcommittee, indicated that full immediate oil decontrol on June 1 would cost consumers $14 billion the first year, increase gasoline prices 4.7 cents a gallon, increase inflation by half a a cent percentage point and the unemployment rate by two-tenths of 1 percent.

The study was released at a Capitol Hill news conference by Rep. Bob Eckhardt (D-Tex.), a foe of deregulation.

In another development, representatives of the nation's governors told Congress that the Carter administration thus far has failed to persuade Americans there really is a serious energy problem.

Speaking for the National Governors Association, Colorado Gov. Richard Lamm and Virginia Gov. John Dalton told an energy subcommittee that stronger signals must come from Washington.

"People are not yet convinced there is a serious problem," Lamm said. "What is needed is a clear declaration by the president that we are facing a shortgage and that unless voluntary conservation is successful, stiffer mandatory measures will follow."

The governors also criticized the administration's standy energy conservation plans -- now before Congress for final approval -- especially the part calling for weekend gasoline station closings.

They said states should be allowed to pick other days of the week, or choose some other alternative for saving energy.

There were these others energy related developments yesterday:

Trans World Arlines said yesterday that because of shar pfuel price rises it will ask the Civil Aeronautics Board for a fuel surcharge of 7 percent in passenger fares and cargo rates across the Atlantic.

If approved, the surcharge would be applied to all tickets purchased on or after April 15 for travel beginning May 1.

A spokesman for the airline said its international fuel prices in the peak 1979 travel season will be 35 to 40 percent above the prices TWA paid in the comparable period last year. If the uncertain foreign fuel price situation stabilizes, the requested fuel surcharge may be the only adjustment required in the foreseeable future, TWA said.

The president of the Edison Electric Institute, an investor-owned electric utility lobby, announced a campaign to seek a change in federal laws and regulations to encourage construction of nuclear and coal-fired powerplants.

Without such action, serious power shortages could occur in the early 1980s, said Bill McCollam, institute president. He said the campaign is an effort "to galvanize the president and the Congress" into action.

Oil industry sources said Mexico is having difficulty meeting commitments for crude oil exports, suggesting that nation's contribution to world oil supplies could be lower than expected next quarter.

According to Petroleum Intelligence Weekly, an authoritative industry newsletter "the exact existent of the shortfall is still uncertain, but it could be as much as 15 percent to 25 percent for some customers."

The Energy Department proposed rules requiring major utility companies to help homeowners select and install energy-saving devices.

The department said that some 64.7 million residences could benefit from application of the program, which was provided for in the National Energy Act passed by Congress last year.