New York City buildings are due for a green makeover, according to a new report that could save their owners a bundle—if they spend some money first.

The Building Energy Exchange has compiled a cheat sheet explaining how long various energy-saving projects would take to pay back a landlord's investment.

Retrofitting Affordability, a blueprint of sorts for the city's 1.5 billion square feet of large multifamily buildings, identified $2.1 billion worth of improvements and found 78% of them would pay back their investment within a decade through energy savings. More than half have a projected payback time of less than five years.

Until the inception of the New York City Energy Conservation Code in 2009, the city followed the state energy code, which exempted renovations made to less than 50% of an existing building's system from having to comply with the modern code. The loophole allowed property owners throughout the city to forgo updates to building equipment to meet new energy-efficient standards. Local Law 85 established the city's code, closed the loophole and mandated equipment updates for all building renovations.

Since then, the city has added requirements to increase the code's stringency, mandating energy audits every calendar year since 2010 and annual benchmarks of data using an online tool created by the Environmental Protection Agency. Stricter energy codes reduced the city's greenhouse-gas emissions by 8.3 million tons from 2005 to 2013.

Rory Christian, the New York director of clean energy at the Environmental Defense Fund, said that landlords are often more concerned about the projects' price tags than their payout, and the $2.1 billion in work scoped out by the report is certainly daunting. However, the potential $350 million in annual savings identified by the analysis could change their minds.

"It's instinctual to think of the cost of doing a new thing," he said. "If you do what [the city] is asking you to do, the savings will significantly offset the costs."

The types of retrofits vary, but many of them don't cost much and would pay back landlords' investment quickly. Pipe insulation and updating energy and building management systems each offer a two-year payback. Yetsuh Frank, managing director of strategy and programs at the Building Energy Exchange, believes that expensive new equipment may not even be necessary.

"A lot can be done by getting existing systems working optimally versus spending on technology," he said.

Owners of postwar buildings that use oil and gas have the greatest potential energy savings from retrofits—14% and 16%, respectively. Aside from the financial benefits, retrofits could improve the quality of life for building tenants.

"A lot of health issues in cities are directly related to dilapidated buildings," said Mr. Frank. "[Retrofits] are a huge benefit for the community as a whole."

Various incentive programs can help landlords pay for retrofits. Con Edison and the New York State Energy Research and Development Authority created the Data Center Efficiency Program, which reimburses up to 50% of the cost of data-center energy studies and provides performance-based financial incentives of up to $5 million per project.

Raya Salter, senior utility advocate at the Natural Resources Defense Council, said that regardless of building size, cost-conscious landlords will avail themselves of these programs.

"Any type of building that doesn't have money lying around will always look for incentives," she said.

Additionally, the Building Energy Exchange plans to hold classes on various retrofitting methods, ranging from simple LED light installation to intensive methods like storm-window installation based on building type. With a host of energy-efficient technologies now available, the hard part can be assessing which methods will work best.

"Ultimately we have to figure out what's the right mix of retrofits in our existing buildings," said Mr. Frank. “We have to figure out what the road map looks like for our city."