“They said there was confusion, and they need to communicate better with the ISO,” McCorkle said.

The ISO spokeswoman emphasized that warnings of any type are carefully issued. Warnings of possible blackouts have particularly serious repercussions, she added.

“We want to be accurate in describing the condition of the power grid,” McCorkle said. “Overuse (of warnings) can lead people to ignore us when we really need conservation and it affects all of California.”

The ISO has established a graduated series of warnings about power supply, which it views as essential to maintaining its credibility and generating response to its calls for conservation.

The ISO alerts begin with Stage One warnings, when reserve electricity-generating capacity falls below 7 percent of demand; Stage Two alerts are issued when reserves slip below 5 percent; and Stage Three alerts are called when reserves shrink below 3 percent.

Stage Three alerts generally trigger planned rolling blackouts, in order to prevent an uncontrolled collapse of the grid from too much demand and too little electricity supply.

The ISO believes its alert system has been the key to generating significant conservation at critical times, including the recent heat wave, when it credits a large volume of public conservation for averting serious problems.

Debra Reed, president and chief executive of SDG&E, conceded there was an error in communication Monday when a utility spokesman told news media of the threat of a Stage Three alert.

“We found one case where one employee made that statement and that was an error,” Reed said. “We are working with the ISO on approved terminology.”

In another case Monday, Eugene “Mitch” Mitchell, SDG&E's vice president of external affairs, called television stations and asked them to warn the public of rolling blackouts if conservation was not immediately implemented. Some stations broadcast the warning.

Mitchell said yesterday that he would use different language in the future, given the same situation.

Reed defended the utility's overall performance during the recent heat wave and denied its announcements had anything to do with winning approval for the Sunrise Powerlink, a controversial, $1.4 billion transmission line that state utility regulators are considering. A round of hearings into Sunrise began Tuesday in San Francisco.

SDG&E says the power line, which would cross Anza-Borrego Desert State Park, is needed to ensure regional electricity reliability.

At various times over the Labor Day weekend, 81,000 SDG&E customers lost electricity because of breakdowns in the distribution systems. Temperatures peaked at about 100 degrees across much of SDG&E's service territory.

At noon Monday, when the warnings were issued by SDG&E, the local electricity system appeared to be headed to a point requiring rolling blackouts, Reed and others said.

The utility officials said regional demand had reached 4,600 megawatts at that time, with a forecast that demand would rise by about 100 megawatts per hour through much of the afternoon. SDG&E had total generating resources of 4,900 megawatts.

That translates to a reserve capacity of about 7 percent. Under ISO guidelines, reserves falling below that point trigger a Stage One alert.

ISO guidelines say a Stage Three can be issued when reserves fall below the required level or are forecast to fall below that level within the next two hours.

As it turned out, SDG&E's demand remained at about 4,600 through much of the afternoon, before falling in the evening.

SDG&E said yesterday that it notified the ISO of the possibility that rolling blackouts might be needed.

McCorkle said the ISO had no such knowledge.