In keeping with their green building philosophy, the owners of Hills Flat Lumber in Grass Valley are installing solar panels for their one-year-old building.

The 30-kilowatt project will handle about 8 to 10 percent of the store's power needs, equivalent to supplying electricity to four to six houses, said General Manager Jeff Pardini.

"It's a long-term investment," Pardini said. "It's a substantial quarter-million-dollar system before the rebates."

Hills Flat expects the system to pay for itself within eight to 10 years, Pardini said.

The company opted for a 30-kilowatt system, because it's the maximum size for commercial rebates from Pacific Gas & Electric Co. at this time, Pardini said.

"We did a 145-kilowatt system in Colfax that takes care of 90 percent of our store there," he said.

More panels could be erected on the Grass Valley store's roof if the new system works out, Pardini said. He doesn't expect energy to get any cheaper.

Plan It Solar of Penn Valley is the vendor on the project, which is being installed by California Solar Electric Company of Grass Valley.

To go along with the new solar panels, Hills Flat already has a geothermal system for heating and cooling, Pardini said.

"Underneath our parking lot we have 24,000 square feet of tubes," he said, where water runs to capture a ground temperature of between 50 and 60 degrees. The water is heated or cooled depending on the time of year and need.

The building also has a computerized lighting system that works in concert with its skylights. Each fixture has six bulbs in it, which run six, four or two bulbs at a time, Pardini said.

"When it senses enough light from the skylights during the day, bulbs shut off," Pardini said.

To contact Senior Staff Writer Dave Moller, e-mail dmoller@theunion.com or call 477-4237.