The Potomac Electric Power Co. has received a $21.4 million rate increase in Maryland, announced a major cutback in its expansion plans and forecast a "significantly reduced" economic growth rate in metropolitan Washington for 1977.

The rate increase, granted by the Maryland Public Service Commission in response to Pepco's request for $36.3 million, means the average Maryland customer will pay an extra $2.45 per month for electricity, the company said.

Pepco received final approval Dec. 16 of a $29.4 million increase for its Washington customers by the District Public Service Commission.

Pepco has 245,300 residential customers in Montgomery and Prince George's counties, with an average monthly consumption of 732 kilowatt hours. The average bill now is $35.05. It will be $37.50, an increase of 7 per cent.

Pepco said it needed the rate increase because of continued rises in costs and continued expectations of greater consumer demands of electricity.

However, the demands are tapering off. Pepco chairman W. Reid Thompson said in a year-end statement that 1976 electricity sales increased only about 3 per cent over 1975. The national sales increase was about 6 per cent, according to the Edison Electric Institute.

In addition, Thompson said, he expected the 1977 rate to fall between 3 and 5 per cent. EEI expects an increase of about 6 per cent.

Thompson said the lower economic growth rate, and consequent lower consumption of electricity, is being caused by "lower population projections, limited building starts, uncertainty over the Dickerson Wastewater Treatment Plant, and the continued effort by the federal government to hold down federal employment . . ."

Pepco is responding, he said, by deferring major expansion of its generating capacity. For the second year in a row, the planned operating date for the company's proposed nuclear power plant, at Douglas Point on the Potomac River 30 miles south of Washington, is being pushed back. Now, Thompson says, the first unit if approved will not go into service before 1987 and the second unit is "deferred indefinitely."

Further, unit No. 4 at the Chalk Point plant in Prince George's County is being pushed back from 1980 to 1981 and unit No. 4 at Dickerson in Montgomery County is being postponed from 1982 to 1983.

Thompson said he regarded the 1976 sales figures as a "continuation of the sharply changed growth trend for electricity which commenced in 1974. From the middle 1950s to the early 1970s Pepco's growth rate resulted in a doubling of sales and peak load every 6 1/2 to seven years . . .

"Beginning with the Middle East oil embargo in late 1973, the Pepco area became a national leader in energy conservation . . ."

However, the Baltimore Gas and Electric Co., which serves the east side of Prince George's and a small part of Montgomery, shows an overall growth in 1976 of 6 per cent, the national average, a spokesman said. The residential usage increase was only 4 per cent, however. A 9 per cent industrial rise pushed up the average.

The Virginia Electric and Power Co. also reported an overall kilowatt hour increase of 6 per cent, but a spokesman said the figures "aren't refined yet."

In citing the Dickerson plant - or the absence of it - Thompson was reflecting the general view that rejection last summer of a Montgomery County proposal for the facility by the Environmental Protection Agency would contribute to a slowdown of area growth.

Most local officials feel the area has inadequate sewage treatment capacity.

Thompson also noted the amount of money Pepco must expend for what he termed "environmental control."

Despite the growth slowdown Pepco intends to spend $590 million on construction in the 1977-79 period, he said. Of that, $89 million, of 15 per cent, will be spent on equipment, designed to protect the environment.