With Mayor Bloomberg declaring a citywide state of emergency to prepare for 100-degree heat today and tomorrow, the chairman of Consolidated Edison, Kevin Burke, is warning he can offer no assurance that the utility’s equipment will not fail again in the coming days as it did last month in Queens.

Forecasters say the oppressive heat that has been blamed for more than 100 deaths in California is headed for the Northeast, with the temperature expected to top the century mark in New York today and tomorrow, according to the National Weather Service.The mayor said yesterday that the projected heat wave had prompted him to sign an executive order Sunday declaring a heat emergency in the five boroughs.

As part of the city’s preparations, the administration will activate its emergency operations center this morning, cooling centers and public pools will stay open for extended hours, and hospitals are girding for an increase in patients. City employees will also have to work in higher-than-usual temperatures under an energy-saving move in which the administration plans to set the thermostat to 78 degrees Fahrenheit in 53 municipal buildings. The summertime temperatures in the buildings are usually set at 72 degrees.

With the power grid already damaged and strained, Mr. Bloomberg yesterday urged New Yorkers to do all they can to conserve energy, and to be careful in the heat. “This is a very serious, dangerous heat wave,” he said at City Hall.

The declaration of emergency came as Mr. Burke testified under oath in the first of a series of public hearings on his company’s response to the blackout that left tens of thousands of residents and businesses in the dark for more than a week.

Seated alone before more than a dozen City Council members and the public advocate, Mr. Burke calmly responded yesterday to more than three hours of intense questioning and pointed criticism at a hearing that gave lawmakers an opportunity to vent frustration over the blackout.The hearing yielded few substantive answers as to why Con Ed’s electric system broke down so extensively in northwest Queens.

Mr. Burke said company officials were analyzing data being gathered from the feeder cables that overloaded during last month’s heat wave. The investigation, along with repairs to the network that failed, could take “many months,” Mr. Burke said. He agreed to demands by lawmakers to provide constant updates to the public.

Council members grilled Mr. Burke on many fronts, but their disapproval centered on Con Ed’s initial response to the outages in Queens, when it underestimated the number of customers affected and gave little indication of when power would be restored.

“When the lights went out, that was just the tip of the iceberg,” said Council Member Peter Vallone Jr., who represents a district in Queens that lost power. Citing “a failure in leadership,” Mr. Vallone said the utility’s response to the blackout was “due to either a pathetic incompetence, which means someone should resign, or criminal negligence, which means someone should go to jail.”

Con Ed initially estimated that fewer than 2,000 customers — each representing three to four people — lost power, and Mr. Burke said the company did not realize the extent of the outage until the fourth day of the blackout. Because of the design of its underground system, the company has no systematic way of determining the size of an outage. Ultimately, officials sent crews out into the neighborhood at night, where they counted homes that had no lights on.

“Clearly the events of the last two weeks have demonstrated that we need a better system for estimating how many customers are without power,” Mr. Burke said in his testimony. In measured tones, he repeated that sentiment several times throughout the hearing, appearing unfazed by the lawmakers’ criticism.

Hailing Con Ed as the country’s most reliable utility, Mr. Burke described a complex electricity network in Long Island City comprising 22 primary feeder cables, a series of transformers, and a secondary network. The system, he said, was designed to withstand the simultaneous failure of two feeder cables, but not the 10 that ultimately broke down.

Although experts have speculated that the outages were linked to Con Ed’s aging infrastructure, Mr. Burke said preliminary reports indicated that the cables that failed were, on average, newer than those that did not.

Under questioning from the council speaker, Christine Quinn, the Con Ed leader acknowledged that potentially thousands of residents in Queens were still relying on generators for power. The utility is keeping its 38 generators in the affected neighborhoods through the week, and some businesses are using their own supplemental power.

Mr. Burke did not back down from Con Ed’s offer of up to $350 in reimbursement to residents for spoiled food, and $7,000 for businesses. Lawmakers say those amounts are far too low and have called on the utility to provide three months free electricity to the affected areas. Mr. Burke said those figures were “reasonable,” but businesses that suffered larger losses should contact the company.

Lawmakers also criticized Mr. Burke for not heeding recommendations from the state attorney general, Eliot Spitzer, after a power outage in Washington Heights in 1999. Mr. Burke said Con Ed had made “dramatic improvements” to its infrastructure and had spent $6 billion on capital projects in the last five years.

Heading into another heat wave, Mr. Burke said there was a higher risk for further outages in Queens, where the network has sustained greater stress and damages.

His testimony yesterday did little to mollify the trio of Queens politicians — Mr. Vallone, Council Member Eric Gioia, and Assemblyman Michael Gianaris — who have been unrelenting in their criticism of Con Ed.

“I ask every New Yorker to conserve and say a prayer, because I have no faith in Con Edison to keep the power on,” Mr. Gioia said after the hearing.

City residents should not expect a reprieve until the weekend. The forecast calls for temperatures to stay at least in the 90s through Thursday before a cold front brings cooler weather on Friday.