WASHINGTON — As 58 million people across 13 states sweated through the third day of a heat wave last month, power demand in North America’s largest regional grid jurisdiction hit a record high. And yet there was no shortage, no rolling blackout and no brownout in an area that stretches from Maryland to Chicago.

But that may not be the case in the future as stricter air quality rules are put in place. Eastern utilities satisfied demand that day — July 21 — with hefty output from dozens of 1950s and 1960s coal-burning power plants that dump prodigious amounts of acid gases, soot, mercury and arsenic into the air. Because of new Environmental Protection Agency rules, and some yet to be written, many of those plants are expected to close in coming years.

No one is sure yet how many or which ones will be shuttered or what the total lost output would be. And there is little agreement over how peak demand will be met in future summers.

The E.P.A. estimates that a rule on air toxins and mercury that it expects to complete in November will result in a loss of 10,000 megawatts — or almost 1 percent of the generating capacity in the United States. Electricity experts, however, say that rule, combined with forthcoming ones on coal ash and cooling water, will have a much greater effect — from 48,000 megawatts to 80,000 megawatts, or 3.5 to 7 percent.