Potomac Electric Power Co. yesterday reported 1985 earnings of $183.6 million ($3.59 a share) on revenue of $1.3 billion, up about 9 percent from earnings of $168.1 million ($3.23) on revenue of $1.2 billion a year ago.

The company said the rise in earnings resulted from slight increases in electricity sales and number of customers served. Part of the increase was attributed to successful marketing of electric heating devices.

*D.C. National Bancorp, parent of D.C. National Bank, has reported annual earnings of $3.76 million ($4.20 a share) for 1985, up 18.6 percent from $3.17 million ($3.94) in 1984.

The bank holding company reported that its fourth-quarter earnings rose 6.6 percent, to $826,000 (89 cents) from $774,900 (83 cents) for the same period a year earlier.

The bank attributed the gains to stabilized interest rates and an increase in the number of loans it has made.

*Some figures for USAir Group earnings were reported incorrectly yesterday. The article should have said that the company reported net earnings of $117.1 million ($3.98 a share) in 1985 on operating revenue of $1.8 billion, down 3.7 percent from 1984 net earnings of $121.6 million ($4.46) on operating revenue of $1.6 billion.

The company, which is based in Washington, said fourth-quarter earnings were up 9.3 percent.