Las Vegas Deputy Director Tom Perrigo wants energy to pay for itself.

The city's sustainability officer plans to install one megawatt ofsolarpanels on 17 municipal parking garages this year and invest $3.5 million in LED and induction lighting to replace 20 percent of Sin City's more profligate street lamps.

He also expects to allocate up to $4 million to retrofit 17 of the city's 100 buildings. The money saved on energy will be plowed back into new energy-efficiency programs. But knowing exactly how much will be saved is no easy calculation.

That's where a separate investment will come in: licensing the four modules of Hara's Environmental and Energy Management software. The software will track the city's $15 million in annual electricity and natural gas use, and establish an accurate baseline against which to measure savings. The more the city saves, the more it can spend on future projects.

But the software will also provide another step forward toward Las Vegas' energy-reduction targets. It will save money through energy monitoring and management.  Perrigo estimates that this will account for savings equivalent to one percent of the city's annual bill, or $150,000, giving the software an ROI of one year.

"It's a major change in the way we do business," says Mayor Oscar Goodman. "It makes us smart."

On Thursday, Las Vegas became Hara's 35th customer, expanding a roster that includes corporations such as Apple, Coca Cola and Hasbro, which it added last month. The Redwood City startup is on the front lines of the rapidly emerging environmental-management market. Its software lets managers track resources going in and out of an organization -- inputs such as energy and water and outputs such as carbon and waste -- and is attracting growing corporate and municipal interest.

Among the company's earliest customers is the Silicon Valley city of Palo Alto, California, which projects savings of more than $2 million on electricity, water and gas bills over the next three years. The savings will come primarily from reduced use.

According to Hara CEO Amit Chatterjee, the company's biggest competitor is the spreadsheet program Excel (though vendors such as SAP, CA and ENXSuite play in the space). Approximately 80 percent of customers record environmental data in Excel before buying Hara, he says.

This includes Las Vegas. The city presently enters data from energy bills and nearly 4,000 meters manually into a spreadsheet. The process is time consuming and often misses anomalies that come up due to equipment failure or billing errors. Responding to these kinds of anomalies will produce $150,000 in savings.

Las Vegas representatives say the city will ultimately move to more sophisticated energy monitoring systems that include sensors and building-management software to examine data in real time. But the systems are expensive and don't easily fit into older buildings. When the buildings are retrofitted, then the new systems can be added.

Until then, Hara is a first step -- and an important one.