CASTRO VALLEY -- The county's newest library soon may not just be lending out books, but power, too.

Construction has started at the Castro Valley Library to make it Alameda County's newest on-site solar power system. Once the solar array is fully operational early next year, it is expected to provide nearly all of the library's electrical needs -- and at times even export excess electricity to surrounding neighborhoods.

"This is a fabulous project," said Carolyn Moskovitz, the library's manager. "This project (not only) used stimulus money in a responsible way to reduce global warming, but also created jobs while making the county greener."

Alameda County has a long history of implementing renewable energy systems at its facilities.

Since 2002, the county has installed 10 on-site solar power systems -- including those at the Santa Rita Jail in Dublin, the Juvenile Justice Center in San Leandro and the Fremont Hall of Justice -- equaling 3.2 megawatts of renewable electricity. The county's goal is to reduce greenhouse gas emissions from county operations 15 percent by 2020.

Matt Muniz, the county's energy program manager, said when completed, the system annually will generate more than 400,000 kilowatt-hours of clean energy and eliminate the library's electrical needs from Pacific Gas & Electric during afternoon hours.

In fact, during most summer days, the system will produce more power than needed and will export excess renewable electricity. The power will be credited back to the county by spinning its electric meter backward, and should make up for any energy used from PG&E during evenings -- when solar paneling does not produce much energy.

"Our goal is to make the library a net-zero building," Muniz said.

The county board of supervisors awarded the project's $1.7 million contract to Sun Light & Power of Berkeley in May. The project is being funded by a 1 percent loan from the California Energy Commission -- funds made available under the 2009 American Recovery and Reinvestment Act.

In addition, over the next five years the county will receive $500,000 from PG&E under its California Solar Initiative Incentive Program. The loan will be paid back during the next 15 years with savings from the solar-generated electricity.

Moskovitz said that on most days the construction work will be done before the library opens so it doesn't distract patrons. She added that solar paneling installation work should go smoothly because it was planned for the library, which opened late last year.

"We were always hoping to do this, so the building was designed with that in mind," Moskovitz said.

She said one of the more interesting features of the project will allow library users to monitor the energy produced and used by the library.

Sun Light & Power will install a monitoring system to show the solar array's performance on a flat-screen monitor in the library's lobby.