There are, roughly speaking, two ways to make a building more energy efficient: change how you use it, or change something about the building itself. Think about this in the context of your home. You could replace the windows, or you could do a better job of turning off the lights.

FirstFuel, a Lexington-based energy analytics startup, has released data this morning suggesting that in commercial buildings, the changes to how energy is used–operational improvements–are a bigger part of potential energy savings than is commonly realized and in many cases require little cost.

“While building retrofits remain the primary focus within commercial energy efficiency, our deep analysis of interval data reveals that a vast savings opportunity still remains largely untouched,” said Swapnil Shah, co-founder and CEO of FirstFuel, in a release. “These findings show that the commercial building sector has a chance to essentially double the potential for energy savings, while simultaneously slashing efficiency implementation costs by a significant margin.”

FirstFuel helps commercial buildings analyze potential energy savings using only software and utility data, as opposed to requiring a person to show up and do an audit on site. (I covered the company’s $10 million Series A in February 2012.)

The data above is based on the company’s virtual audits of 60 million square feet of commercial space across a diverse array of buildings. Of the savings opportunities that the company’s modeling identified, just over half were from operational changes:

The FirstFuel analysis revealed that 51 percent of all energy efficiency opportunities from the sample – representing approximately $12 million in total savings – could be achieved through low and no-cost operational improvements. When extrapolated to the entire U.S. commercial building market, the total savings potential for operational improvements represents a $17 billion opportunity.

To be clear, in the context of commercial buildings, this isn’t just about turning off the lights (although that can be part of it, usually with the help of intelligent lighting systems). It includes things like not running the heating or cooling systems when nobody is in the building, or making sure the two aren’t running at the same time. Both the software and hardware to more efficiently run commercial buildings is itself a promising area for energy innovation.

This data is interesting in part because there is some debate about the size of the energy efficiency opportunity. It’s generally believed to be a major win-win opportunity as it tends to pay for itself while making the economy more efficient and lowering the greenhouse gas emissions that contribute to climate change. But economists at MIT have raised doubts about the magnitude of such win-win opportunities. While FirstFuel is by no means a disinterested party, it’s contribution to the field is nonetheless interesting. To the extent that the energy efficiency opportunity exists, it may be easier to capture– through operational improvements rather than retrofits–than previously thought.