Virginia utility regulators today awarded the Cirginia Electric and Power Co. a $148.1 million rate increase that will boots bills paid by most residential customers by an average of 11 percent.

Coupled with the most recent previous increase, the action by the State Corporation Commission means Vepco's residential rates will have increased 20 percent in nine months. The company said its one million customers will be paying the highest average electric rates on the East Coast South of Philadelphia as a result of the increase.

Today's increase will bring the company's average residential bill to about $50 a month from $45.21, according to Stanley P. Ragone, Vepco president.

Ragone told a news conference today that the increase is not enough and said the company soon will be back before the commission for another rate review.

"Present customers will enjoy the benefits of today's ruling but only at the expense of higher energy costs in the future," he said.

Regone said the company will ask for another rate increase in about a month to cover the costs of operating a fourth nuclear power plant and will begin an analysis that could result in another general rate increase request.

In addition to future increases the company will seek, Vepco customers have been warned that they face an immediate rise of as much as 9 percent in charges because of the shutdown of the Surry nuclear power plant by U.S. safety regulators.

The Nuclear Regulatory Commission closed Surry and three outher nuclear generating units last week because a design error may have resulted in cooling systems too weak to withstand earthquakes.

If that shutdown lasts for more than three weeks, Vepco officials have said its customers will have to pay an additional $11.5 million a month in fuel charges to cover the costs of using more expensive coal and oil supplies to generate electricty. This could translate into an added monthly expense of more than $2.50 for the average residential customer.

Today's award of $148.1 million is about 60 percent of the $246 million sought by the company in a case that began more than a year ago. The total increase is the largest ever granted to a utility in Virginia.

As a result of today's order, Vepco charges to all customers will be about 16 percent higher than they were on June 1977. The commission agreed with Vepco that residential bills should go up an average of 20 percent because homeowners and appartment dwellers with individual meters have not been paying a fair share of electricty costs.

The SCC also backed Vepco contentions that residential customers are paying only about 80 percent of their fair share of costs while small subiness customers are paying 20 percent more than their share. The share now paid by the largest commercial customers is about right, the SCC ruled.

The first step of the increase given final approval today came in an $82 million temporary boost last June 6. The company has been collecting it through an 8.5 percent surcharge on all rates. The additional $66 million -- a 6.8 percent increase in current carhes -- will be collected beginning April 1.

At that time, the company will reallocate its charges among all customers to place a greater burden on residential consumers.

The commission considered a proposal formulated but not recommended by Vepco that would have reduced the bills of approximately 48,000 welfare receipients by more than 23 percent. This "financial assistance rate," also called a "Lifeline rate," was rejected in favor of continuing a policy requiring all classes of customers to pay their share of electrity costs.

The SCC noted that a General Assembly committee is studying the question of adopting special rates for the poor through legislation.

Vepco officials agreed that the SCC order translates to a 20 percent increase in average residential rates during the last 10 months. However, they said actual bills may not rise that much because of lower fuel charges made possible by use of nuclear power plants.

The $50 million increase that will be southt to coincide with the opening of North Anna II nuclear plant in midyear more than likely will be offset by fuel cost savings, Ragone said.

Fuel cost decreases and increases were passed on to customers automatically each month until the end of last year. Now these changes in fueal charges are approved separately by the SCC and adjusted after quarterly hearings.

In its 78-page opinion today, the three-member commission cited testimony by its technical staff that seven of Vepco's largest generating plants have operated less efficiently than most similar plants elsewhere. Ragone disagreed with this finding but said the company will cooperate with what the commission called an "affirmative action program" to improve its operations.

Consumer spokesmen initially offered cautions praise for the SCC order, a reaction that apparently was based on the assumption that residential users would not bear a disproportionate share of the increase.