C3, which provides corporate energy management software, just scooped up Efficiency 2.0, a residential energy efficiency company that uses rewards to encourage people to save energy.

For C3, which was founded by Siebel Systems’ Tom Siebel, the acquisition allows the company to bring more comprehensive offerings to utilities that want to tackle all of the customer classes.

Efficiency 2.0, which is based in New York City, recently picked up Southern California Edison as a customer, and also counts ComEd, Northeast Utilities and Cambridge Energy Alliance as some of its clients.

The appeal of Efficiency 2.0 is its low-cost solution compared to other hardware-intensive options. That is also the appeal of one of the largest players in the residential space, Opower. Indeed, the similarities between the two firms' mailed reports were so similar that Opower filed a lawsuit against Efficiency 2.0, which was settled last month based on "a mutually agreeable resolution of this matter," according to the companies.

What Efficiency 2.0 does that no one else does is offering cash rewards for saving energy. The customers who just receive mailed reports save about 2.5 percent, while those that log into the web program to earn rewards save just over 6 percent.

The system is working. Tom Scaramellino, Founder and CEO of Efficiency 2.0, estimated in April that his company would save the carbon equivalent of 25 million pounds of CO2 in 2012, and he expected that figure to quadruple next year.

“C3 and Efficiency 2.0 have been independently experiencing very rapid growth,” Ed Abbo, president and CTO of C3, said in a statement. “Our respective customers have been very clear that they require an integrated energy efficiency SaaS solution to meet their pressing energy mandates. This business and technology combination is in response to market demand.”

C3 has been expanding in all directions in 2012. Earlier this year, the software-as-a-service company announced a partnership with SAIC to provide a more “fully integrated energy solution” for C&I customers. The San Mateo, Calif.-based company also has a partnership with HP.

Currently, most energy management companies target either commercial or residential customers, with others focusing on small commercial. The appeal of one-stop shopping will mostly likely be cost, especially as some utilities are mandated to drive down demand.

"Through this combination, C3 offers utilities a single source for residential, small/medium business and large commercial and industrial energy efficiency solutions,” Saul Zambrano, Sr. Director, Products Group, Customer Energy Solutions at Pacific Gas and Electric Company, said in a statement. “The C3 SaaS solutions will help utilities achieve their energy savings targets and increase customer satisfaction across all customer segments at a low total cost and risk.”

Utilities will be looking to keep costs down as they adopt customer-facing energy management programs, but it is unclear just how powerful a driver one-stop shopping will be. Many utilities are already working with various vendors to provide demand response platforms, commercial energy efficiency and residential engagement platforms.

However, for the municipal and co-op market, which are even more price sensitive than their investor-owned counterparts, the appeal of one company able to deliver on all of these needs is clear. The acquisition shows that the appeal of residential energy management is not just growing, but becoming a necessary part of the portfolio for utilities.

The details of the deal between C3 and Efficiency 2.0 were not disclosed.