With their predictable customer base and high yields, electrical utilities are often considered among the dullest investments. That's changed.

Their large dividend payments make them ideal for complex trading strategies, and the result has been an explosion in volume. The hot summer, resulting in heavy use of air conditioning, has increased earnings projections for many by as much as 10 percent, said Barry Abramson of Prudential Bache Securities.And the entire industry may be on the verge of the kind of change that may have little impact on next quarter's earnings but profound implications over the next decade. A looser regulatory environment and the emergence of competitors could inspire a wave of mergers. Multi-billion dollar construction projects that bedeviled the industry in the late 1970s and early 1980s are now mostly concluded, raising the prospect of increased cash flow.

And finally, conservation is waning. The demand for power, which dropped off early in the decade, is on the rebound, a trend fraught with expensive pitfalls as well as the potential for far-larger profits.

In the post-crash era of uncertain financial markets, large utilities have been among the better securities to have. When both dividends and capital appreciation are taken into account, the 21 utilities included in the Standard & Poor's 500, dramatically outperformed the broader index during the latter part of 1987. They faltered during the spring market rally, but they have moved upward since May, according to Smith Barney, Harris Upham & Co. Inc.

Alan Berro, a portfolio manager at Fidelity Investments, said that during the early 1970s industry expectations for growth in power usage hovered around 5 percent, but following the two oil shocks and the tendency of people to conserve energy, a 1 percent to 3 percent growth rate appeared more likely. Recently, however, hot summers, cold winters and other factors have pushed demand to the 4 percent to 5 percent range once again.

That has a mixed impact for utilities. Initially, utilities should benefit from higher demand, but ultimately they may be forced to once again engage in the expensive process of building new generating plants, an ordeal they once thought would be avoidable.