WESTBOROUGH —For patrons at the D'Angelo sub shop on Turnpike Road, the commotion going on a few yards away at the Car Wash and Detailing of Westborough last month must have raised a few questions. After all, what could a car wash possibly be building on its roof? Skylights?

"Everybody seemed to be asking about it," said the car wash and detailing shop's owner, Bruce Forrestall.

But now that the project is complete, drivers coming westbound down Rte. 9 can clearly see that what Forrestall has built on his business's roof is a large arrangement of solar panels. When they're switched on - Forrestall is hoping to hold the grand reopening of his business on Oct. 21 - the building's electricity bill will be reduced 20 percent, courtesy of the sun."I just thought, with the cost of electricity now, we'd try to get energy efficient," Forrestall said. The project - which cost about $150,000 - is one of the largest renovations in the business's 18 years.

"The rates increased almost 50 percent (this spring), and I use a lot of electricity for the car wash," he said. "How am I going to cut electricity?"

His answer was to cut wasteful use of energy. Inside the car wash he replaced all the ceiling lights with new, more economical fixtures. He also replaced his old pumps with more efficient ones.

And he started looking into solar power. He found that, despite the steep capital expense, he could recoup his investment in five years, thanks in part to state and federal incentives.

The most enticing government break came from the state. Working with SolarFlair Energy of Hopkinton, Forrestall was able to secure rebates for 40 percent of the project's initial cost through the Commonwealth Solar Program, which helps residents and businesses go solar. At tax time the business can also claim a one-time solar tax credit to slice another 30 percent off the total cost.

The state's solar program was launched in January.

Another side benefit to solar power is that the system will generate more electricity than is actually needed at some times of day. The surplus can be sold back to the utility.

Not every business can plop down $150,000.

"The capital investment is significant," Forrestall said, but he wondered why so few other businesses in the region have taken advantage of what he considered a pretty good deal, considering the escalating cost of electricity.

"(It) really surprises me," he said. "I don't think people know about it."

Emily Dahl, a spokeswoman for the Massachusetts Technology Collaborative, the state's economic development agency for renewable energy, said plenty of businesses have been showing interest. The solar program has already helped get 300 projects started across the state, she said.

A couple of projects are in the works locally, including one at a manufacturing business in Framingham.

Aside from significantly cutting his electricity costs, Forrestall said the solar system also allows his business to operate in a more environmentally friendly way, which in itself is an advantage for a business in a time when "green" is fashionable.

"We haven't really been promoting it," he said.

He said he believes there could be some car owners who would appreciate "a car wash company that's trying to save the environment.

"I'm hopeful," he said.