Mayor Marion Barry's proposal to save $12 million by making utility users pay the District's street lighting costs ran into trouble yesterday as city officials acknowledged they weren't sure how the plan would work and power company representatives said it would be illegal.

The plan also "amounts to double taxation," said council member Jerry A. Moore Jr. (R-At Large), chairman of the transportation and environmental affairs committee, during council hearings on Barry's $1.99 billion operating budget for fiscal 1984. "I consider it a sneak attack" on taxpayers, Moore said.

Currently the city pays Pepco for the cost of electricity used in city street lights. When Barry introduced his budget on Jan. 10, he proposed to cut $12 million from the Transportation Department's budget by having Pepco add the cost of street lighting to the regular electric bills it sends customers throughout the city.

City officials estimate that the plan would increase the average homeowner's monthly bill by 83 cents and that of the average business by $16.

Thomas M. Downs, director of the Department of Transportation, said yesterday that the city only last week asked a private law firm that specializes in regulatory issues to determine how the city could implement the plan.

"It's a question of equity," Downs said. "Everyone who is a utility user in the District benefits" from street lighting, he said. Currently, however, the federal government, embassies and other private, tax-exempt organizations do not pay taxes used to pay the cost of street lighting.

Alan Kirk, vice president and general counsel for Pepco, told the council that his firm was not consulted on the plan. "We are opposed to it," Kirk told a reporter after his testimony before the council committee. "It's impractical, unfair and inequitable. It would discriminate against other customers," Kirk said. "They would be paying for someone else's electricity."

Kirk said similar plans have been rejected in other jurisdictions.

Also at yesterday's hearing, council member Hilda H. Mason (Statehood-At Large) criticized plans to cut the city subsidy for student Metro fares.

"That is discrimination [against] students, to require them to go to school and charge them a full fare," she said. Subsidized fares are 20 cents a trip compared with the 65-cent rush-hour fare.

Downs countered, however, that more than half the 105,000 school students would still be eligible for reduced fares and that those with the ability to pay full fare should be required to do so.