Potomac Electric Power Co. yesterday reported profits of $5.3 million (11 cents a share) for January, down from $6 million (13 cents) the same month a year ago. Operating income for the month increased from $11.5 million to $10.8 million.

For the past 12 months, Pepco reported earnings of $84.2 millions ($1.79) up from $77.6 million ($1.74), as operating income grew from $555.6 million to $668.2 million.

First Virginia Bankshares Corp., a Falls Church bank holding company, has reported a 12 percent increase in operating profits for 1977.

Not counting gains or losses from selling securities, First Virginia earned $9.6 million (91 cents a share) compared with $8.5 million (81 cents) in 1976. Total net income was $10.2 million (97 cents) compared with $8.5 million (81 cents), according to Chairman Ralph A. Beeton.

In a fourth quarter alone, the banking company's operating profits were up 9 percent to $2.4 million (23 cents a share) from $2.2 million (21 cents) in the final quarter of 1976.

As of Dec. 31, deposits at the company's 20 banks througout the state were $1.11 billion, an increase of 10 percent over the previous year. Loan volume rose by 15 percent of $746 million.

A.H. Robins Co., a pharamaceutical manufacturer based in Richmond, reported flat earnings for the fourth quarter and lower profits for all of 1977.

In the final three months, Robins earned $6.9 million vs. $7.1 million, or 27 cents a share in both periods, although sales rose to $84 million from $76 million.

For the full year, Robins earned $26.8 million ($1.03 a share) on sales of $307 million compared with profits of $31 million ($1.20) and sales of $285 million in 1976.

Telenet Corp., a Washington data communications common carrier, reported revenues of $4.06 million for 1977 compared with $1.2 million the previous year.

Expansion of the firm's data net work and development of new communications processors resulted in a net loss for the year of $4.1 million compared with a 1976 loss of $3.7 million.

As of Dec. 31, Tele net had 180 corporate, government and university computer centers as customers for its international network. Telent completed its first offering of stock to the pulic last December.

E.C. Ernst Inc., an electrical contracting and engineering firm in Washington, reported record profits of $2.4 million ($1.25 a share) in the nine months ended Dec. 31 compared with $2.1 million ($1.14) a year earlier. Contract revenues rose to $101 million from $96 million.

In the third quarter, Ernst earned $557,000 (29 cents) on contract revenues at $37 million compared with profits of $490,000 (25 cents) and revenues of $32 million in the same period during 1976.The backlog of contrcat work on Dec. 31 was a record $174 million, up 79 percent from last year, Chairman Edward P. Johnson said.