Potomac Electric Power Company yesterday asked for an 18.9 percent increase in electric rates for its Maryland customers.

If the Maryland Public Service Commission approves the request, Pepco's customers in the state will pay an additional $95.5 million a year in electric bills.

Pepco said the full increase would boost the average Maryland customer's bill by 34 cents a day -- from $53.50 a month to $63.70.

The power company said the sharp jump in electric rates is needed primarily because of high interest rates that Pepco has to pay on loans to finance its power plants.

"Prolonged high interest rates have a major impact on us because of the large amounts of capital we must raise to finance new facilities for customers and refinancing maturing low-cost borrowings," said Pepco Chairman W. Reid Thompson.

Pepco has a request pending in the District of Columbia for a 9.6 percent increase in D.C. rates that would cost customers $37.1 million a year.

In the District, Pepco is asking for authority to earn a profit of 10.6 percent of the amount it has invested in plants, lines and facilities.

In Maryland, Pepco asked for an 11.73 percent rate of return on its investment in facilities, up from the current 9.52 percent.

Thompson said Pepco needs to charge higher rates in order to attract investors to finance its growth. U.S. Treasury bonds pay investors 15 percent interest, so utility companies must pay 17 percent or more, he said.

Pepco's last Maryland rate increase was granted last May and boosted rates by only 2.7 percent.

Since 1977 base rates have increased only 9.5 percent, Thompson noted. The base rate, however, does not include the increases in the cost of fuel, which are passed through to customers automatically.

Including fuel, Maryland electricity costs have increased about 25 percent since 1977, from 4.8 cents a kilowatt hour to 6 cents an hour.

Thompson said the 25 percent boost gives Pepco "a strong record of holding down price increases" because the Labor Department's Consumer Price Index has jumped 48 percent in the same period.

Pepco's rates have gone up less than most other utilities, because the company has had to build fewer new plants than other power companies, Thompson said. Because its customers are conserving power and the growth electricity consumption in the Washington metropolitan area has slowed dramatically, Pepco has chopped $1.5 billion from its construction program in recent years.

Between now and 1985, Pepco plans to ask utility regulators in D.C., Maryland and Virginia for basic rate increases totaling $521.6 million, D.C. People's Counsel Brian Lederer said in a report last week to the D.C. Public Service Commission.

He said Pepco's projections are that fuel costs will jump by another $326 million by 1985.