The Potomac Electric Power Company's ability to generate 43 per cent more electricity than anyone wanted in 1976 cost its customers an unnecessary $26 million, according to a consumer oriented list of dubious honors released today.

The Environmental Action Foundation, a Washington-based group that picked the nation's "Dirty Dozen" industrial polluters two years ago, ranked the nation's 100 largest investor-owned electric companies in 15 categories it said were growing worries of consumers. Pepco ranked fifth in annual cost to consumers of excess generating capacity.

The Edison Electric Institute, an industry trade group, immediately denounced the "Utility Scoreboard" as a "a laundry list of unreasoned complaints (that) reflect a basic lack of knowledge of the electric utility industry and of common business and accounting procedures."

Richard Morgan, director of the two-year study, told a press conference the "Scoreboard" booklet was designed as a collection of utility data that usually is "hard to find and even harder to understand." Figures are those reported by the utilities to government agencies, he said.

Excess capacity to produce power costs ratepayers money in idle machinery, he said. "We have to come up with guidelines on how much reserve capacity customers can be expected to pay for."

A Pepco spokesman said the company had revised its construction plans downward three times since conservation trends became apparent after the 1973 oil embargo. Still, he said, peak demand last summer exceeded predictions. The Edison Electric Institute said last summer's demand was 6.5 per cent above 1976 levels and set nationwide records.

"The anticipated continued growth of demand in the coming years strongly emphasizes the necessity to construct new generating plants and other essential facilities now," the institute said.

Consolidated Edison Co. of New York and Commonwealth Edison Co. of Chicago, both among the nation's largest companies, ranked or tied for first place in three "Scoreboard" categories each. Con Ed led the field in residential billing rates (8.78 cents per kilowatt hour, twice the national average), charitable contributions ($3.5 million) and air pollution violations (eight tied with Ohio Edison Co.).

Pepco was listed No. 10 in executive salaries with the $158.687 paid to board chairman and president W. Reid Thompson. The compnay also was ranked third in air pollution violations, with six plants in noncompliance with Environmental Protection Agency standards.

Pepco vice president Horace Webb said only five plants and not six were in noncompliance with the standards, but noted that $165 million will be spent in 1978 to install pollution control equipment. Negotiations are under way to revise the standards in Maryland so as to allow current emission levels, he said.

The Environmental Action Foundation reported that the Virginia Electric and Power Co. charged customers $45.8 million in U.S. taxes in 1976 but paid only $1.9 million to the federal government. The foundation attributed this alleged "overcharge" of $43.8 million to a federal law permitting utilities to avoid passing savings from tax "loopholes" on to consumers. The foundation said Vepco ranked 14th in the nation in the size of the "phantom tax" charged its customers.

The study also said Vepco was 12th highest in penalties paid for pollution control in 1976, 16th highest in executive salary paid, 24th in amount donated as charitable contributions and 38th for advertising and public relations expenses.