NEW YORKERS have often been told that they use less energy than most Americans, partly because they live in the most densely populated city in the country.

And that’s true, up to a point.

Sure, New Yorkers have the benefit of an extensive mass-transit system, which means lower auto emissions, but the city’s residential buildings are less energy-efficient than those in many other places in the country, particularly in eco-friendly states like California and Vermont.

“The main reason that New Yorkers use much less electricity is that our apartments are so much smaller” than homes in other cities, said Rohit Aggarwala, the director of the Long-Term Planning and Sustainability Office, part of the Mayor’s Office of Operations.

In fact, most big New York buildings, both commercial and residential, are wasting thousands of dollars a year on energy, the city says. Energy use by buildings accounts for almost 80 percent of the city’s greenhouse gas emissions, and residential buildings for about a third of that. These gases are released in creating the energy used to heat, cool and light the buildings, as well as to run myriad household appliances and gadgets.