Sometime next year, utility regulators in Maryland will decide one of their biggest rate cases on a seemingly simple premise: Should consumer ratepayers or utility stockholders pay for the extended shutdown at the Calvert Cliffs nuclear power plant?

If the outage was caused by mechanical problems, the 1 million customers of Baltimore Gas & Electric Co. must pay. If it was the fault of BG&E management, the stockholders must shoulder the financial burden, which has been mounting at the pace of nearly $600,000 a day.

Virginia regulators are grappling with a similar issue. One unit of Virginia Power Co.'s nuclear plant at Surry was shut down in the summer of 1988 for repairs that required nearly a year, forcing the company to increase its purchases of electricity from other utilities. The staff of the State Corporation Commission is studying the cause of the shutdown to determine whether the company or the ratepayers should bear the costs, a company spokesman said.

If the question before regulators in the two states seems simple, the implications of the answer are vast. BG&E says the tab for buying replacement electrical power stood at $220 million through February. By some estimates, however, the ultimate cost could be twice as high before both generating units at Calvert Cliffs are again producing electricity.

With such amounts at stake, a major battle is taking shape at the Public Service Commission's Baltimore headquarters, where volumes of testimony are flowing in from the giant utility, major industrial customers such as General Motors Corp. and Bethlehem Steel Corp. and from the Office of the People's Counsel, a state-appointed attorney who represents consumers.

"One thing we know for sure: The customers didn't cause this," said John M. Glynn, the people's counsel, arguing that the company and its stockholders should cover costs associated with the idled plant.

John Metzger, a BG&E spokesman, said the outage "was not caused by management problems but equipment. If that's the case, that amount should be recoverable through the rate structure."

The Calvert Cliffs case is an example of the regulatory commission's typical quandary: choosing a line between what is best for utilities and what is best for customers in Maryland. But there is a magnified difference. "There are few cases of such dollar size that they get much public attention," said Frank O. Heintz, chairman of the five-member commission.

Even while the commission began grappling with the replacement-power case, BG&E last week filed a separate action seeking a 12.1 percent base rate hike to cover higher operating and maintenance expenses, plus capital costs.

Calvert Cliffs, located at Lusby, about 40 miles southeast of Washington on the Chesapeake Bay, has arguably been good to BG&E customers and stockholders, functioning well and holding down electric rates for Baltimore and central Maryland since it opened in 1977. A recent nationwide study of electric rates of 187 utilities found BG&E's near the lowest third.

Problems at Calvert Cliffs began in 1988, with regulatory fines imposed by the Nuclear Regulatory Commission, its placement on an NRC "watch" list of troubled plants and the accidental death of a worker, Gary Proffit. In May of 1989, Unit Two was shut down after cracks were discovered in a critical pressurizer. Operation of Unit One also was halted for inspection. BG&E, saying that no similar structural problems exist in Unit One, expects to restart it in coming weeks. Unit Two will be down for most of 1990.

While Calvert Cliffs is out of commission, BG&E must purchase more expensive electricity from other utilities. This replacement power cost is at the center of the brewing storm at the Public Service Commission.

The commission's decision, by some calculations, could mean a difference of 10 percent to 20 percent on a BG&E customer's monthly electric bill. A decision that goes against BG&E, on the other hand, likely would affect its stock and its already-declining bond ratings, causing the utility to spend more money for borrowed funds. BG&E stock, which has traded at between $28 and $34.87 1/2 in the last year, closed yesterday at $28.37 1/2, unchanged.

Glynn, noting NRC fines against BG&E and inspection reports criticizing management practices at Calvert Cliffs, said state law is clear that customers shouldn't pay for what he considers company mistakes. But he is concerned that the commission will try to find a middle ground.

"The commission tries to rule like a New Testament God rather than an Old Testament God," Glynn said in an interview. "Sometimes people should be punished for their sins."

To recover their additional fuel costs from the Calvert Cliffs debacle, BG&E had filed a series of rate requests with the commission and had begun charging consumers higher rates. But in an interim ruling issued earlier this month, the commission cited a "cloud of doubt" about the causes of the outage and signaled that BG&E will have to carry some of the financial consequences, at least for the time being.

The commission, in a rare split decision, ordered the utility not to pass along to customers automatically a portion of the added fuel-purchase costs. For BG&E's service territory -- which also includes Anne Arundel, Howard and small portions of Prince George's and Montgomery counties -- this could mean small savings beginning June 1. Depending on whom is interpreting the ruling, it could cut the fuel portion of bills by from 75 cents to $1.89 a month for a typical residential consumer.

The majority was quick to say that the interim decision could be modified or reversed when the commission issues a final decision next year. Yet it noted in its reasoning: "This commission cannot be oblivious to the fact that the {NRC} has, at a time overlapping the outages at Calvert Cliffs, indicated that deficiencies existed in the operational and procedural systems."

Staff writer Thomas W. Lippman contributed to this report.