HONOLULU — Hawaii is first state to require solar water heaters in new homes.

Gov. Linda Lingle signed a bill into law Thursday requiring the energy-saving systems in homes starting in 2010.

“This solar power legislation is another important step in our long-term plan for energy independence in Hawaii,” she said.

Hawaii relies on imported fossil fuels more than any other state, with about 90 percent of its energy sources coming from foreign countries, according to state data.

The new law prohibits issuing building permits for single-family homes that do not have solar water heaters. Some exceptions will be allowed, such as forested areas where there are low amounts of sunshine.

State Sen. Gary Hooser, vice chairman of the Energy and Environment Committee, first introduced the measure five years ago when he said a barrel of oil cost just $40. Since then, the cost of oil has more than tripled.

“It’s abundantly clear that we need to take some serious action to protect Hawaii because we’re so dependent on oil,” Hooser said. “I’m very pleased the governor is recognizing the importance of this bill and the huge public benefits that come out of it.”

Hooser, D-Kauai-Niihau, said the environment and the economy will benefit.

Opponents of the law have argued that the mandate is unfair and will drive up home prices in the islands, where a modest home can start at $600,000.

Hooser said buyers will actually save in the long run, with monthly savings of at least $50 off their electric bill. With thousands of new homes built in Hawaii every year, that’s millions of dollars in savings a year, providing a boost to the economy.

“I’m hoping this will start a trend nationwide,” said Hooser, who plans to meet with lawmakers in other sunshine states, such as California, Nevada, New Mexico and Arizona to push similar laws.

Hawaiian Electric Co. estimates about 85,000 households, or roughly 20 percent of all homes in the islands, already have solar water heating systems.

They come standard in many new homes being built by developers.

“It’s one of the best things a homeowner can do,” said Darren Pai, spokesman for the utility.

A traditionally powered water heater is the largest consumer of electricity inside a home, accounting for about 25 to 40 percent of the power used. HECO currently offers a $1,000 rebate on solar water heater installations. In addition to the rebate, federal and state tax credits reduce the cost of a typical $5,200 system to about $1,500.

HECO said it normally takes just a few years to recoup the costs in Hawaii, which has the nation’s highest electricity prices. The utility supported the measure and worked closely with the Legislature.

Lingle signed the bill but expressed concerns with some of the provisions and language, including ending the current state solar tax credits beginning in 2010 worth about $2,500 in savings to buy a solar water heater. The bill also appears to eliminate the tax credit for owners of existing homes after Jan. 1, 2010.

While it may not have been the intent of the bill, the language implies that existing homeowners would have had to seek a building permit to install a solar water heater prior to Jan. 1, 2010, to claim the tax credit, Lingle said.

The governor said her administration will introduce legislation to clarify the language and ensure the tax credits will continue.

In January, Lingle announced the Hawaii Clean Energy Initiative partnership that aims to have at least 70 percent of Hawaii’s energy needs supplied by renewable sources by 2030.