Potomac Electric Power Co.'s earnings grew 18.8 percent during the second quarter because of higher electricity usage and higher rates to customers, a company spokesman said yesterday.

Pepco posted profits of $21.1 million (38 cents a share) compared with $18.1 million (32 cents) last year. Earnings for the first half increased from $36.7 million (66 cents) last year to $41 million (73 cents) this year.

For the 12 months ended June 30, Pepco earned $114 million ($2.20) compared with $106 million ($2.09) in the corresponding period last year. During that time, earnings grew 5.3 percent and operating expenses, including fuel costs, grew 13.1 percent.

Second-quarter revenues increased 10.7 percent to $242.1 million from $218.7 million last year. Electricity usage was up 2.4 percent.

First-half revenues went from $440.4 million to $496.1 million; 12-month revenues, from $949.6 million to $1.056 billion.

Pepco, which most recently was granted rate increases in Maryland in April, the District in December and Virginia in May, has a $88.5 million rate increase pending in the District..

Maryland National Corp., a bank holding company that owns Maryland National Bank, opened 14 new offices during the second quarter, which, along with slower loan growth, cut into Marynat's earnings, company officials said yesterday.

For the three months ended June 30, the Baltimore firm earned $9.3 million ($1.16 a share) compared with $9.4 million ($1.23) last year. Assets of the holding company were $4.3 billion, up from $3.8 billion at the end of June 1981.

For the first half, Marynat posted profits of $19.6 million ($2.47), down slightly from $19.8 million ($2.59) the year before.

Black and Decker Manufacturing Co. yesterday reported lower sales and earnings for its third quarter ended June 27, attributing the decline to poor chain-saw sales and foreign currency losses.

Officials of B&D, based in Towson, said profits fell 61 percent to $4.5 million (11 cents a share) from $11.9 million (28 cents) last year. Sales dropped 10 percent from $331.8 million to $298.2 million.

B&D posted nine-month earnings of $32.2 million (77 cents) compared with $50.3 million ($1.20) a year earlier. Sales declined by 10 percent to $1.01 billion from $1.12 billion.