The Empire State Building has long been a signature feature of Manhattan's skyline. But owners of the iconic edifice, planning a $1 billion initial public offering, hope to convince investors and tenants that it's what's on the inside that really counts.

The 81-year-old New York tower has undergone a massive, 3-year makeover designed to cut energy use, modernize office suites, and attract tenants willing to shoulder higher rents. Building owners and key suppliers on Monday will detail the energy savings they've achieved so far.

While the new One World Trade Center has claimed the title of New York's tallest from the Empire State, the 102-floor Art Deco building towers over many others when it comes to the world of building retrofits.

The Empire State is the highest-profile project in a growing collection of renovations that are becoming big business for industrial conglomerates and electrical service firms.

As America's towers show their age, especially in the Big Apple, where nearly half of office space was built before 1945, companies like Honeywell International Inc, Johnson Controls Inc, Siemens AG and United Technologies Corp are eyeing a retrofit market predicted to generate $16 billion in annual revenue by 2020, up from about $5 billion last year.