The state Department of Transportation is moving forward with a plan to turn the rooftops of its airport and harbor buildings into mini-electrical generating units, announcing it had selected Hoku Scientific Inc. for the work.

Hoku will design, engineer and install photovoltaic power systems at four airports and at two Kaua'i facilities that will generate up to 779 kilowatts of electricity. The purchase-power agreements with Hoku call for the Kapolei-based company to sell electricity to the state at an predetermined rate over the next 20 years.

The state also announced it hoped to award purchase-power contracts for Honolulu Airport by the end of the year and that it is planning to issue a call for proposals on other Department of Transportation sites next year.

The contract award with Hoku culminates months of work by the state, which has a long-term goal of making Hawai'i the renewable-energy showplace of the nation. Now the state gets about 85 percent of its energy from petroleum, and rising crude oil prices have contributed to Hawai'i's economic problems.

The Transportation Department started soliciting bids last December as state officials looked for ways to cut rising electricity bills. Hoku's contract covers 10 photovoltaic installations involving at airports division buildings at Lihu'e, Kahului, Kona and Hilo, along with a baseyard facilities buildings for the highways and harbors division on Kaua'i.

"Now, more than ever before, the state must seek every opportunity to integrate renewable power and energy conservation into public operations," said department director Brennon Morioka in a statement.

The 779 kilowatts is expected to offset up to 15,000 tons of carbon dioxide emissions over the lifetime of the systems.

Hoku said it plans to complete the installations by the end of the year pending the obtaining of third-party financing.

The contract announcement by Hoku was the second this week and follows one on Wednesday that it was selected to install a 34-kilowatt photovoltaic system on the Xcel Buildding in Hale'iwa.