The Massachusetts Institute of Technology and NStar said they have renewed an energy-efficiency agreement that has a goal of cutting energy usage on the school’s Cambridge campus by 21 million kilowatt-hours a year.

MIT and NStar said they have just concluded the first phase of a program called “MIT Efficiency Forward.” Its objective was to reduce campus energy use by 15 percent, a saving equivalent to the electricity used by 4,500 Massachusetts homes. Based on that experience, MIT and NStar decided to move ahead on the second phase of the program.

“For this second phase, Efficiency Forward will now also include thermal savings from the reduction of natural gas use on campus, with a goal to save 150,000 therms of natural gas annually over the next three years,” the two said in a press release.

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According to Israel Ruiz, MIT’s executive vice president and treasurer, the program has not only resulted in energy efficiency, but it “has also allowed us to use our campus as a living laboratory to test new ideas.”

The release added: “Since 2010, work through Efficiency Forward has touched nearly every building on MIT’s campus and started to influence the way the MIT community thinks about and uses energy. The construction of high performance, energy-efficient buildings such as the Sloan School of Management – which beats the baseline building energy code by 45 percent – as well as the installation of energy efficient lighting and refrigeration; HVAC upgrades with advanced thermostats; and compressed air system consolidation have contributed to the success of the program.”

NSTAR, a Northeast Utilities company, transmits and delivers electricity and natural gas to 1.4 million customers in Eastern and Central Massachusetts, including more than 1 million electric customers in 81 communities and 300,000 gas customers in 51 communities.