To help with potential electricity shortages this summer in New York City, businesses and building owners controlling more than 300 million square feet of office space in the city said yesterday they’d take voluntary steps to reduce their energy use. The participants in the program, which covers about 75 percent of the office space in the city, said they would begin reducing their power demand ASAP, raising average building temperatures by 2 degrees Fahrenheit and turning off unnecessary lights — saving an estimated 25 to 30 megawatts. In an emergency, they said they could drop demand by 160 megawatts by taking such steps as reducing elevator service and dimming lights near windows.