Peak summer electrical use -- a key measure of power demand use -- has dropped for the first time in nearly 40 years.

According to a survey by the Edison Electric Institute, an industry association, peak demands for electricity fell 2.7 percent last summer, a stunning reversal for the utility industry, which once could count on steady 7 to 9 percent annual growth in markets. The last time such a decline occured was 1945.

The recession and industrial shutdowns appeared to be a major factor in the decline, with conservation and moderate weather also reducing demand. According to a Dean Witter Reynolds Inc. industry analysis, the Midwest and other industrial areas have registered the greatest declines in demand, with Duquesne Light of Pittsburgh reporting a drop of nearly 15 percent. Detroit Edison has reported no growth in peak demand in 10 years, the analysis noted.

Since January of 1982, overall power generation has decreased by 2.1 percent and sales have dropped 1.6 percent, according to Carl Tobie, manager of statistical analysis for the EEI. The industry's reserve margin, or surplus generating capacity, is 38.7 percent, the largest since 1938.

Peak demand is significant because utility companies must design their generating capacity to provide for that period, however brief, of maximum use. Reports of a decline in electrical use come amid a sharp debate over whether the utility industry is overbuilding.

Nationally, since 1975, 116 proposed power plants (85 of them nuclear and 31 of them coal or oil-fired) have been canceled. The canceled capacity is more than 96,685 megawatts, or more than the combined generating capacity of Texas and California.

A draft Department of Energy study on the future of the electrical utility industry has said that there are "danger signs" that the nation might be left dangerously short of electricity, but utility critics say that the demand for more building simply reflects a continuing underestimation of the power of conservation by the industry and its friends.

Noting that electric utilities had predicted a 2 to 3 percent increase in the summer peak for 1982, rather than the decline that occured, Environmental Action Foundation economist Richard E. Morgan called the new figures "evidence of continuing wave of conservation by electricity consumers.

"When you see how much consumers have conserved without much help from their utilities, it makes you wonder how much energy we could save if the utilities really tried to encourage efficiency improvements," he said.

EEI's recent estimate of a 2 percent increase in electrical sales next year may have to revised because of indications that economic growth in 1983 will be slower than anticipated, Tobie said.