NEW YORK (CNNMoney) -- The Empire State Building is on an energy diet.
The hulking building, a symbol of American power and, to some, excess, has cut its energy use by 20%.
And that's just due to changes to the building's exterior. Once retrofits are made to tenant spaces on the inside, the second tallest building in Manhattan will be nearly 40% more efficient.
The retrofits will cost $20 million once they're complete, and are expected to save the owners $4.4 million in annual energy costs.
"After one year, we have proven that investing in energy efficiency gives building owners a dollars-and-cents advantage," said Dave Myers, a president at Johnson Controls, which conducted the retrofit.
The renovations are part of a $500 million rehab plan for the building. The building's owners, Malkin Holdings LLC, filed for an initial public offering back in February which valued the building at $2.5 billion.
The changes to the Empire State include:
--Filling the existing windows with an energy saving gas and adding an additional plastic pane.
--Upgrading the building's cooling system.
--Using computerized "smart" energy management technology that can adjust temperatures floor by floor.
--Provide tenants with detailed energy use in their space.
--Automatically shut off lights in unused areas.
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