San Mateo public works officials said Monday they are finally ready to install solar panels atop the library, 4½ years after the facility opened. They began advertising the construction contract this week and expect work to begin in April and finish by August.

City leaders figured they would buy the rooftop panels after residents approved a $35 million bond measure and raised another $10 million in donations to construct the facility last decade. But the cost of construction climbed to $65 million, and the 93,000-square-foot center opened in August 2006 without the panels.

"We just didn't have enough money," City Librarian Ben Ocon said. "It's always been something that we would like to have added."

But now the library has all the money it needs, having recently received a $400,000 grant from the U.S. Department of Energy for the 70-kilowatt system.

"The library is sort of the symbolic living room of the city of San Mateo, and it's important for it be a landmark of sustainability," said Councilman David Lim, who has advocated for green initiatives. "It says a lot about where we want to go."

Even with a traditional roof, the library earned a LEED gold rating from the U.S. Green Building Council and was the first facility to earn the county's green business designation, Ocon said. The library, which is touted as being 20 percent more energy-efficient than state guidelines, features lights that turn off automatically during sunny days and a large rooftop tank that stores cool water at night for daytime use.

But solar panels, which have become one of the strongest symbols of the green movement, may make the biggest impact. The city expects the system to cut out more than 50 tons of carbon dioxide each year and save about $20,000 annually on utility bills, even before it applies for state rebates that would double that amount.

"It's certainly something we feel good about," Ocon said. "It was envisioned as a green building, and as a library it's great to be in that position to showcase that type of technology here."

The city will still have to use electricity to power most of its building, but the solar panels should provide enough energy for at least 11 percent of the facility.

About 2,000 patrons visit the book hub at Third Avenue near El Camino Real each day, including many from Hillsborough, and they will also see a solar panel information kiosk at the library's "Going Green" area once the project is finished.

It's also fitting for a city that now plays home to the nation's leading solar provider, SolarCity, which just moved its headquarters from Foster City to an office on Clearview Way.

Ocon said since the work will be on the roof, he does not expect there to be any changes to library hours or services during the project.

The library was the city's first green building project, and has since been followed by the environmentally friendly police station.

The Department of Energy grant, for which the city applied in 2009 as part of the federal stimulus program, also included $325,000 to replace some streetlight bulbs with LED lights and $110,000 for residents to get energy audits for their homes.

Contact Mike Rosenberg at 650-348-4324.