New customers, higher electric rates and greater use of electricity by the growing Metro subway system helped push Potomac Electric Power Co.'s second-quarter profits to $24.96 million (45 cents a share) from last year's $21.05 million (38 cents).

Second-quarter revenues of the power company increased more than five percent to $255.15 million from $242.13 million in the same period last year, and first half revenues increased to $508.9 milion from $496.1 million.

Pepco earnings of $47.61 million (85 cents) for the first six months of 1983 were up from last year's first half of $40.99 million (73 cents), in part reflecting its 4.3 percent rate increase in Maryland in February.

First Virginia Banks Inc. reported a 19 percent jump in second-quarter earnings and attributed the increase to a 13 percent growth in assets and higher interest rates. Black & Decker Manufacturing Co.'s profits plunged as the result of a corporate restructuring and the sale of its chain-saw business, but higher earnings were reported by Evaluation Research Corp., Ryland Group Inc., Farmers and Mechanics National Bank and Central National Bank of Maryland.

First Virginia earned $7.67 million (61 cents) for the second quarter, compared to last year's second-quarter net of $6.45 million (52 cents). Earnings for the first half of the year were up 25 percent to $14.84 million ($1.18), compared to net income of $11.8 million (95 cents) for the first six months of 1982.

John Hanson Savings and Loan posted record earnings of $4.34 million ($2.32) for its 1983 fiscal year ended June 30, a complete turnaround from last year's loss of $1.09 million(-57 cents). The Beltsville savings association also set a company record with fourth-quarter profits of $1.62 million (86 cents), compared to a fourth-quarter loss of $20,161 (-2 cents) last year.

Evaluation Research Corp. announced second-quarter earnings of $306,000 (10 cents) an increase of 105 percent from last year's second-quarter net of $149,000 (8 cents). Profits per share increased only 25 percent due to issuance of 1.2 million new shares after the purchase of Electromagnetic Technology Inc.

Posting the highest net earnings of any quarter in its 16 years, Ryland Group Inc. showed second-quarter profits of $4.88 million (78 cents), up from last year's second quarter of $605,000 (10 cents), adjusted for a two-for-one stock split.

Ryland's earnings reflect a huge increase in home sales which were up 184 percent from last year.

Black & Decker's third quarter fiscal earnings for 1983 dropped drastically compared to the same period last year, after the company took big write-offs for the sale of its gasoline chain saw business and a major corporate restructuring. Third quarter earnings plunged to $317,000 (1 cent) from last year's $4.5 million (28 cents) in the same period.

Earnings for the first six months of 1983 for Maryland's Farmers and Mechanics National Bank rose to $2.1 million ($3.06) from last year's earnings for the first six months of $1.7 million ($2.48).