(CNN) Just days after a power outage left part of New York City in darkness for hours, a major heat wave has officials eying the city's electric grid warily, urging people to stay cool with air conditioners -- but with restraint.

To be clear, utility officials have said last weekend's outage , while still under investigation, wasn't caused by excessive demand.

Still, the outage is a reminder that electricity isn't always a sure thing, and Mayor Bill de Blasio has asked residents and businesses not to go overboard on air conditioning, so that the electrical grid isn't too strained.

Even the governor has concerns. During an interview with Long Island News Radio 's Jay Oliver on Friday morning, Gov. Andrew Cuomo was asked if he had confidence in New York utility company Con Edison after last week's power outage.

"Am I confident? No," Cuomo said, laughing.

"Am I all over Con Ed, am I doing everything I can? Yes. ... And do I have people monitoring Con Ed, and do I have an alert system where if there's a problem, I will be on it in 11 seconds? Yes."

People cool off near the fountain at Washington Square Park on a hot Wednesday afternoon in New York City.

Mayor asks residents not to overdo air conditioning

New York City, like many big cities across the East and the Midwes t, is under an excessive heat warning on Friday. That kind of warning is issued when the combination of heat and humidity is expected to make it feel like it is at least 105 degrees for a prolonged period.

De Blasio on Thursday declared an emergency for the city from Friday to Sunday evening, ordering certain tall office buildings to have their thermostats set at no lower than 78 degrees.

"The city government is limiting its energy use to reduce strain on the electrical grid, and now private office buildings will also have to do their part."

State officials issued an air quality health advisory for metro New York and areas of Long Island for Saturday and Sunday.

"When outdoor levels of ozone are elevated, going indoors will usually reduce your exposure," officials said through a news release. "Individuals experiencing symptoms such as shortness of breath, chest pain or coughing should consider consulting their doctor."

A heat wave is on the way, New York City. Take it seriously — we could see some of the highest temperatures in years.



Head to https://t.co/B0DWoVpTzh for more information. pic.twitter.com/hFvRAOFn4K — Mayor Bill de Blasio (@NYCMayor) July 18, 2019

Weekend is best time for heat wave, power company says

New Yorkers are still not trustful of Con Ed after Saturday's outage, and the mayor said he has asked the utility six or seven times for a reason as to why the blackout happened.

Speaking to host Brian Lehrer on radio station WNYC on Friday, de Blasio said Con Edison has not given him "good enough answers," but told him overload was not it.

He said the company told him an overload is not predicted for this weekend, but residents should cut back out of an abundance of caution.

The president of Con Edison in New York, Tim Cawley, said Friday that conservation would indeed be good this weekend.

"It's good for the environment; it lowers bills; and in a period like this, it just takes a little bit of the edge off the demand on the system," Crawley said at a news conference.

A heat wave coming on a weekend might be better for utilities than facing one on a weekday. New York's peak energy consumption happens on weekdays, and although power demand still is high on weekends, the grid can handle that amount, Cawley said.

"We're ready for what the heat will bring," Cawley said. He added, however, that peak demands this weekend may "rival all-time weekend peaks."

Last weekend's outage is blamed partly on a faulty relay protection system

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Last weekend's outage started Saturday at 6:47 p.m., and the lights were back on shortly after midnight, officials said. It mostly affected midtown Manhattan and parts of the Upper West Side. No injuries or hospitalizations were reported.

At the height of the outage, 72,000 customers were in the dark, Con Edison said.

On Sunday, Cawley said customer demand when the outage happened was low, and that his company was confident the outage had nothing to do with high demand.

A day later, the utility released preliminary findings: That a substation's relay protection system didn't operate as designed

There was a problem with a distribution cable, and the relay protection system didn't contain the problem, but rather allowed it to knock out several electrical networks, the utility said

The incident still is under investigation.

Con Edison spokesman Alfonso Quiroz told CNN affiliate WABC that the utility is ready for this weekend.

"We've got over 4,000 crew members on the ground, ready to respond to outages as they occur," he said.