Beginning Jan. 1, 2010, builders in Hawaii will no longer be able to use conventional hot water heating systems in their homes. Solar, or some other energy-efficient source, will be required.

Hawaii Gov. Linda Lingle recently signed into law a bill requiring that all homes built after January 1, 2010, be equipped with solar or other energy-efficient hot water systems. Though the bill allows for other options, the new law is widely seen as a solar hot water mandate and is expected to cut home energy usage in Hawaii by an average of 30% starting in 2010.

"Hawaii is almost totally dependent on imported oil for its energy needs and estimates show that, with this law, our oil consumption will be cut by 30,000 barrels during the first year and continue to decline exponentially thereafter," according to a statement from the office of Senate Majority Leader Gary L. Hooser, who introduced the bill.

Hawaii currently has the highest electricity costs in the nation, Hooser says, adding that it is estimated that homeowners will save $600 annually for a family of four. "The additional disposable income combined with a cumulative multiplier effect of that income circulating in the Hawaii economy, rather than being exported to import foreign oil, will result in significant additional economic activity," the lawmaker says.

Solar water heating systems typically include solar collectors on a roof or some other structure and storage tanks located in the house. Though available in many configurations, systems use heat from the sun to either preheat water before sending it to a conventional heater, or a one-tank system in which the back-up heater is combined with the solar storage in one tank.

A solar water heating system is not always the obvious home run that most expect, however. Yes, the systems rely less on fossil fuels, but they can be pricey. According to the Department of Energy's Office of Energy Efficiency and Renewable Energy, solar hot water systems usually cost more to purchase and install than conventional water heating systems and the long-term savings depend on a variety of factors: the amount of hot water used; a system's performance; geographic location; available financing and incentives; and the cost of conventional fuels, among others.

But the climate is ripe for energy savings. Hawaii's new law, among other things, shifts existing state solar installation tax credits to homes built prior to 2010; and with record-high gas prices, homeowners are likely to see a faster return on investment.

"Mandating solar hot water heating for all new homes is a no-brainer," Hooser's statement says. "This is the low hanging fruit, a low-cost, proven technology that saves homeowners money and is great for the environment."

Nigel Maynard is a senior editor for BUILDER magazine.