Aided by this summer's record-breaking heat and the Washington area's rapid economic growth, the region's electrical utilities have been setting their own records, forcing the companies to take another look at their generating capacity.

On July 21, Virginia Power hit a maximum electrical usage of 11,529 megawatts, roughly 12 percent higher than last year's record level. On the same day, Pepco reached a high of 5,153 megawatts, 9.6 percent greater than its previous record.

The rise in maximum electricity usage, called peak demand, was more than triple Pepco's expectations.

For Pepco, which serves the District and its Maryland suburbs, the greater demand means that the company might have to buy additional capacity from other utilities or build a new power plant sooner than anticipated. Virginia Power, which serves the Virginia suburbs, is also studying the implications of the figures.

For several years the growth in peak demand has been fueled by rapid economic growth in the area. The increase has been accelerated this summer by extraordinarily hot weather that has caused customers to crank up air conditioners.

The failure of two of Pepco's transmission lines on Wednesday -- forcing the utility to cut service to some downtown offices for an hour -- was unrelated to the heavier than expected demand, Pepco spokeswoman Nancy Moses said. The outage was due to "typical faults of underground cable," she said. The utility, which on Wednesday had asked downtown customers to curtail their electrical use, rescinded that request yesterday.

While Pepco says that it has more than enough capacity to meet the requirements for electrical power for the forseeable future, its prediction in 1985 that peak demand would grow only between 1 and 2 percent a year has been substantially off. In 1984, demand growth jumped 4.8 percent, and in 1985 it jumped by 4.3 percent. In 1986, growth in peak demand remained steady, but it took a substantial jump again this year.

"The Washington area has grown with such rapidity that they can't even meet normal demand," said Franklin Salisbury, a vice president of the District consulting firm of Energy-Tech Inc. "They have to go to outside power grids."

Salisbury said the utility failed to anticipate the explosion of growth in the Washington-Baltimore area. "People think of the energy crisis as {related to} foreign oil, but here's one in which the utility never anticipated the demand."

Peak demand for electricity in Virginia Power's service territory has also grown faster than expected. The utility had predicted a 3 percent annual increase in peak demand over the last five years, but has instead seen growth in peak demand of roughly 5.4 percent a year.

Virginia Power's sales of electricity for the first six months of 1987 were 8.5 percent higher than for the first six months of 1986, a year when the company's sales grew faster than all other utilities east of the Mississippi River. Pepco's sales grew only slightly the first six months of this year.

Pepco customers' July bills were up about 7.5 percent over last July due to increased consumption, and they used 13 percent more electricity. Virginia Power customers are using roughly 10 percent more electricity this summer than they did last year, but the utility could not estimate how much more they are paying.

Officials at both utilities are taking notice of the growth. Pepco has turned to Ohio Edison, where it has purchased the right to 450 megawatts that the Ohio utility does not need.