The coal strike already is adding from 80 cents to $1.15 a month to the electric bills of Potomac Electric Power customers and ultimately will mean higher bills for Virginia Electric Power Co. customers, executives of the two utilities said yesterday.

Dwindling coal supplies have lead Pepco, which services the District of Columbia and the Maryland suburbs, to cut back coal consumption and start burning more oil.

Oil costs 50 percent more than coal and the $1.5 million a month increase in fuel costs will show up on customers' bills two months from now, said Horace Webb, Pepco's director of corporate affairs.

Even if striking miners approved proposed contract settlement, promptly, the higher bills will continue for at least two more months until coal stockpiles are rebuilt, Webb added.

After that, customers will face higher bills because the new coal contract will mean higher coal prices, predicted a spokesman for Vepco, which serves the Virginia suburbs.

Vepco has felt the impact of the strike less than Pepco, because it uses less coal. Vepco normally gets 26 percent of its power from coal, 36 percent from oil, 24 percent from nuclear plants and the remainder from other sources. Coal is Pepco's main fuel, producing about 70 percent of its power.

The Vepco spokesman said there has been "a tendency for an increase" in its customers bills, because the utility had to buy extra coal to stockpile for the strike and to pay higher prices for it.

To conserve its coal - now at a 60-day supply - Vepco has raised the output of its nuclear power facilities, which currently are supplying 29 percent of its power.

But if the strike continues, and Vepco's supplies shrink to the 40-day level, the company will have to increase its oil consumption, meaning higher costs for consumers.

Pepco stockpiled a 100-day supply of coal before the strike and is now down to 60-day stocks at present consumption levels.

At its big Morgantown, Md., generating plant, which can burn both coal and oil, Pepco is using 60 percent oil and 40 percent coal. That plant normally gets 85 percent of its fuel from coal.

Only two of the five generators at Pepco's coal-burning plant on the Potomac River are in operation now to save fuel, Webb said.

There is no danger that a prolonged coal strike will result in power shortages here as it already has in some middlewestern areas, he added. Pepco has several plants which burn only oil and if necessary could convert its Chalk Point generators from coal to oil.