Young Brothers Ltd., the state's largest provider of interisland barge service, is looking to the sun to cut electricity costs.

The company has signed a 20-year contract to buy electricity from a unit of Hoku Scientific Inc., which will install a 224-kilowatt rooftop photovoltaic system on a Young Brothers building at Pier 39 and 40 at Honolulu Harbor.

Young Brothers is the latest Isle business to turn to renewable energy sources to lower electricity costs. Hawai'i's commercial electricity customers pay the highest rates in the nation, with the 21.92 cents per kilowatt hour cost at the end of 2007 more than twice the national average of 9.67 cents, according to the U.S. Energy Information Administration.

The barge company won't be financing, installing and maintaining its own system, though. Instead the company will buy the energy from Hoku under the purchase agreement. Hoku plans to install the system this year subject to obtaining financing.

"It certainly does give us a great degree of stability for our future energy costs," said Roy Catalani, Young Brothers vice president.

Catalani said the contract provides for some savings on what Young Brothers currently pays Hawaiian Electric Co., with negotiated increases over the life of the contract. The system should provide 34 percent of the power used by Young Brothers refrigerated cargo and maintenance facilities at Honolulu Harbor.

The remainder of the power will continue to be provided by Hawaiian Electric. Catalani said Young Brothers and Hoku may expand the contract to include photovoltaic systems for other buildings in Honolulu and the Neighbor Islands.

He said Young Brothers and affiliate companies have been and are continuing to look at other ways to lower its fuel and electricity costs. This includes using electricity off Hawaiian Electric Co.'s grid for tug boats instead of running diesel engines while in port.

Tug boat propulsion systems are 10 percent more efficient and faster after exchanging traditional propellers for a system employing nozzles and high efficiency propellers. As part of a barge replacement program, Young Brothers is switching to more fuel-efficient hulls that also allow the company to carry more cargo.

"We have an obligation to control our fuel costs," Catalani said. "We are doing what we can to meet that goal."

Hoku, a Kapolei-based company that focuses on clean-energy technologies, also has a contract to install a 218-kilowatt photovoltaic system and sell power to Hawaiian Electric. The system is to be installed this year under a 20-year agreement between the companies.