The school year began Sept. 5, but the solar electricity passive generation systems being built on several school roofs did not begin with it.

“Since we do not yet have the final details of the legal agreement resolved, I can’t be definite as to the completion date,” said Christopher Burney, the school district’s and town’s facilities director.

“Safe to say that we would like all of the installations complete before the winter,” Burney said.

The work on all three systems is being done by the same parties, Burney said.

The project, including Middlebrook School and Miller-Driscoll School, is expected to save $2.1 million over 20 years, and now Wilton High School has been added, creating an additional 249 kilowatts of alternating current power.

The project at the high school’s annual generation is 361,150 kilowatt hours. The annual savings in the electric bill is projected to be $30,727, and over 20 years, the savings will be $570,000, just from the high school.

It is the latest chapter in the town’s movement toward being green. On May 8, the town entered an agreement with the solar company known as Kingspan.

Connecticut Green Bank will technically own the system. Kingspan will build it, and the system owner will subcontract the operation and maintenance to another company.