BOSTON ( MainStreet) — Scientists at the Department of Energy's Pacific Northwest National Laboratory have say that extra-efficient rooftop units that heat and cool commercial buildings can reduce energy costs an average of about 41% compared with most units in operation today.

Not only that, but if such units immediately replaced all those existing rooftop units that have a cooling capacity of 10 to 20 tons, it would save $1 billion annually.

The DOE looked at a commercial rooftop HVAC unit known as the "Daikin Rebel," made by Daikin Applied — one of the winners of the DOE's Rooftop Challenge manufacturers to create a rooftop unit far surpassing current DOE efficiency standards. ( Carrier was the other winner). It's part of the DOE Rooftop Campaign, which promotes energy-efficient rooftop units.

Nearly 40% of all greenhouse gas emissions in the United States come from the design, construction and operation of buildings. Typically, rooftop units use up a significant amount of the 18 quadrillion BTUs of energy the average commercial building burns each year.

Back in 2009, the consulting firm McKinsey & Co. report that showed that the U.S. could reduce its non-transportation energy consumption by nearly a quarter (23%) by 2020 and save $1.2 trillion by just by maximizing energy efficiency. The landmark report called a focus on energy efficiency, particularly in the commercial building sector, low-hanging fruit in achieving significant cuts fast in greenhouse gas emissions.

All existing rooftops adopting the winning Rebel model over the next decade would be like taking 700,000 cars off the road or shutting down eight coal-fired power plants each year, said the DOE scientists who conducted the Rooftop Challenge study, Srinivas Katipamula and Weimin Wang.

"There are great gains waiting to be made in energy savings using technologies that exist today," said Katipamula, whose research was supported by DOE's Office of Energy Efficiency and Renewable Energy, in a press release.

Comparing the Rebel's efficiency to models in operation today using simulation software, they found it would cut energy demand by 15% in Chicago, 37% in Houston and 36% in Los Angeles — and cut energy costs by 33%, 44% and 45%, respectively. The model even exceeded new energy reduction regulations and outperformed newer, efficient units that meet current regulations by several percentage points.

The costs of these units were not factored into the DOE analysis, but Katipamula estimates it would take at least a few years for the technology to pay for itself. Other factors could also affect energy savings, including local or regional climate, the size of a building, its insulation and building materials.

Since the DOE did not look at features other than cost that could add to the investment, though, such as energy saved for heating, it could pay back sooner than expected. Katipamula believes that offering government incentives would be one way to help offset the initial cost of an energy-efficient Rebel model and persuade builders to invest in the technology.