Continuing its impressive performance under airline deregulation, Washington-based USAir yesterday reported increased earnings for June, the second quarter and first half.

And Potomac Electric Power Co. reported improved results for the quarter, half and 12 months ended June 30, and increased its dividend.

USAir's June earnings totaled $7.8 million (50 cents a share fully diluted), up 15 percent from $6.8 million (43 cents) in June 1979. Second-quarter earnings set a single-quarter record for the airline at $21.6 million ($1.38), which was up 17 percent from the $18.6 million ($1.23) recorded in the same period of 1979. During the first half, USAir earned $26.7 million ($1.70) compared with $11.6 million (81 cents) a year earlier.

"We continue to be encouraged by the relative strength of our primary markets," Edwin I. Colodny, USAir's chairman and president, said yesterday.

The airline's 1980 performance comparisons to last year are especially significant because USAir's second-quarter 1979 results were favorably affected by the strike against United Airlines.

Operating with a philosophy of controlled growth under airline deregulation, USAir added routes to some sunny, warm points to balance its traditionally seasonal Northeast route, enabling it to report a profitable first quarter for the first time in the ariline's history.

In yesterday's report, USAir said revenues totaled $83.6 million in June, up 30 percent from $64.3 million for June 1979. Its revenues for the second quarter were up 31 percent to $243.7 million, while those for the first half were up 39 percent to $455.1 million.

Last month, USAir said it was paying 88.3 cents a gallon for fuel compared with 52.4 cents a gallon in the same month of 1979.

Pepco's board of directors increased the quarterly dividend for common stock by 3 cents a share to 38 cents. The higher dividend will be payable Sept. 30 to sharelholders of record Aug. 28. Regular dividends for preferred and preference stock will be payable Sept. 1 to shareholders of record Aug. 1.

Chairman W. Reid Thompson said the company's earnings improvement, "though still considerably short of the return authorized by the regulatory commissions, gives us the opportunity to provide a needed dividend increase to our investors."

Three-month net income rose from $13.8 million (26 cents a share) a year ago to $17.7 million (33 cents) while operating revenues increased from $166.4 million to $175.2 million; six month net income went from $26.3 million (50 cents) to $34.9 million (50 cents) while operating revenues went from $340.1 million to $346.8 million; and 12-month net incomes was $92.9 million ($1.88 a share) on operating revenues of $756.1 million, up from net income of $78.4 million ($1.65) and operating revenues of $745.6 million for the previous 12 months.