It was so hot Wednesday that City Hall issued a 24-hour heat advisory warning people to stay indoors — but officials also cautioned against blasting the A/C.

New Yorkers should keep their air conditioners at 78 degrees to help reduce the risk of blackouts, Mayor de Blasio said.

“If the air conditioning is set at 78 degrees, people are safe and they will be able to deal with the hot temperatures,” he said.

A 75-degree setting uses 18% more electricity, and a 72-degree setting uses 39% more, he said.