Saying its costs are rising faster than its revenues, Virginia Electric and Power Co. yesterday requested a 12.2 percent increase in its rates, and said it would seek an additional 8 percent increase later this year.

The 12.2 percent temporary rate increase would go into effect April 20 if approved by the utility-regulations State Corporation Commission. The dollar increase in Vepco customers'bills would vary according to how much power a family uses and whether its house is all-electric. The additional cost for an all-electric home - where a bill now is about $115 to $150 monthly, depending on the season - will range from about $14 to about $18.

Vepco said the rate increase would raise its revenues by $118 million annually. The utility said the second increase - which it expects to seek in the spring - would yield an additional $80 million.

"We are acutely aware of the impact of the increased energy cost on customers," Vepco president Stanley Ragone said in a statement, "but inflation and rising costs have drastically increased the cost of producing electricity. The plain truth is that we can't hold down the price of electricity when the cost of generating it continues to go up."

The chief reason for the 12.2 percent increase that would go into effect April 20 is Vepco's newest nuclear power plant, North Anna, a $750 million facility that is scheduled to start producing electricity the same day. In an extensive advertising and public relations program, Vepco has argued that nuclear energy, which accounts for 26 percent of the power it generates, is far cheaper than energy produced from oil-or coal-fired power plants.

But the apparent paradox has an explanation, a Vepco spokesman said. Starting April 20, he said, the utility's customers will have to start paying through their basic rates, the multi-million-dollar interest cost on the money Vepco borrowed to build the plant, as well as other costs associated with its operation.

However, he said North Anna will save Vepco customers about $50 million annually that would be paid through the fuel adjustment clause added to monthly bills if the plant had used oil or coal instead of nuclear energy. Ragone said in his statement that the 12.2 percent rate increase actually amounts to 7 percent - or $68 million annually - because of the $50 million saving on fuel.

Vepco's rate increase request comes 14 months after the utility won its last increase (8.5 percent) and at a time when the General Assembly is considering a number of bills that would eliminate or modify the fuel-adjustment clause through which customers are billed monthly for any fluctuation in the price of fuel used by Vepco at its power plants.

"Immediate rate relief is essential," Ragone said; "to prevent the company's financial condition from worsening dramatically when North Anna Unit I goes into service."

Last year the company, the largest electric utility in Virginia, reported that the rate of return on its investment for the year that ended last June 30 was about 8.9 percent - below the 9.6 percent return it is authorized by the SCC.

The company said its stock continues to sell at a substantial discount below book value at a time when most other electric utilities have recovered" from the increases in the mid-1970s in the cost of fuel and operating and construction expenses.

The proposed rate increase did not come as a big surprise to consumer representatives since the utility had announced it would seek some rate relief this year. But John T. Schell of McLean, who has represented various consumer groups in Vepco rate cases since 1972, said: "There is no question that Vepco's need for revenue is caused by its own construction program. To some extent there will be justification for raising additional revenue. But the North Anna nuclear plant was delayed for a year, at great cost, because of Vepco's troubles with the Nuclear Regulatory Commission. The delays were the company's own fault, and it shouldn't recoup because of those delays."

Though Vepco has cut back on its construction program - units 3 and 4 at its surry nuclear station were canceled after millions of dollars had been spent on planning - it still has a sizable construction program planned.

Vepco aaked the SCC for the 12.2 percent temporary increase "without a hearing or upon an abbreviated hearing" - something the regulatory agency has done in the past when the firm's rate of return has fallen below the ceiling level projected by the SCC.

In another rate case, Potomac Electric Power Co. yesterday slashed in half its request for a rate boost in Virginia. W. Reid Thompson, testifying before the SCC, cut the original request for a $3,187,000 increase to $1,573,000.

He told the commissioners the company's financial condition had improved since the original request was filed several weeks ago. He also indicated some reductions may be possible in Maryland and the District of Columbia, where Pepco also has rate cases pending.