The District of Columbia people's counsel recommended yesterday that the Potomac Electric Power Co. delay operation of its controversial oil-fired Chalk Point generating unit until 1990.

D.C. people's Counsel Brian Lederer said in an interview that the proposed eight-year postponement of the Prince Georg's County generating plant would allow time to determine whether the unit, known as Chalk Point No. 4, should be oil-fired, converted to coal or some toher fuel or, possibly, scrapped altogether.

In his most detailed analysis of Chalk Point, Lederer said in a letter to the D.C. Public Service Commission that the proposed delay of Chalk Point No. 4 -- combined with other recommended changes in Pepco's plans -- could mean a saving of $25 million to $45 million for Pepco customers.

A Pepco spokeswoman said late yesterday afternoon that Pepco officials had not yet received Lederer's recommendations and were unable to comment on them.

The nearly $200 million, 630-megawatt unit has become increasingly controversial because the federal government is urging electric utilities to switch plants from costly oil to cheaper coal. Chalk Point No. 4 is expected to be amoung the last major oil-fired units built in the United States.

Lederer's recommendations yesterday were based on an enginnering study conducted by a consulting firm, R. W. Beck and Associated of Phoenix. As people's counsel, Lederer represents consumers in preceedings before the rate setting commission.

To offset the proposed postponement of Chalk Point No. 4, Lederer recommended that Pepco start operating another new generating unit -- the coal-fired Dickerson No. 4 in Montgomery County -- at an earlier date than currently planned. Pepco has scheduled Dickerson No. 4 for 1978. Lederer proposed that it begin operating in 1985 or 1986.

In addition, Lederer proposed that Pepco rehabilitate several existing District of Columbia units at its Benning Road and Buzzard Point plants. Pepco has announced plans to shut down these units, saying they have become obsolete. h