A cash rebate for Hawaiian Electric Co. customers who install solar water heaters will double to $1,500 for a limited time in an effort to lift sagging sales of the energy-efficient units.

The rebate, which had been $750, will be available for systems purchased between Monday and May 31, or until program funds run out, said Hawaii Energy, the company that administers the state’s energy conservation program. The company did not say how much money is available for the program.

The enhanced rebate, combined with a 35 percent state tax credit and a 30 percent federal tax credit, will reduce the cost of a typical solar water heating system from $6,600 to $1,785.

Hawaii Energy said it is boosting the rebate to lift sluggish sales of solar water heating systems. Rebates given out for solar water heaters fell to 3,656 in 2010 from 8,770 in 2009, according to Hawaii Energy.

Program officials said a bold move to increase the rebate was needed to get people’s attention.

"We’re glad to be able to offer these bonus rebates to help residents take advantage of the benefits of solar water heating in these tough economic times," said Ray Starling, program manager for Hawaii Energy. "Solar water heating significantly reduces electricity usage and helps to lessen our state’s dependence on imported oil."

Solar water heaters, which can cut water heating costs by up to 90 percent over an electric resistance heater, are considered one of the most cost-effective ways to cut a household power bill. Once installed, a system will pay for itself in two to four years depending on electricity rates, according to Hawaii Energy.

The standard $750 rebate is paid for by a fee levied on Hawaiian Electric Co. ratepayers. The increased rebate is funded through a federal stimulus grant.

The offer is available on Oahu, the Big Island and in Maui County.

Homeowners also will have the option of combining the old $750 rebate with a new $1,000-interest-free loan. That program, also funded with federal stimulus money, allows participating financial intuitions to make interest-free loans to homeowners for solar water heaters.

The rebates are not available for solar water heaters installed on new homes. A state law that went into effect Jan. 1, 2010, requires most new homes to be built with solar water heaters.

With about 80,000 solar water heaters installed, Hawaii leads the nation in the number of solar heating systems on a per capita basis, according to the Hawaii Solar Energy Association. Over the past three decades the penetration level for residential single-family homes has grown to about 30 percent, the trade group said.

However, the penetration rate should be even higher given Hawaii’s abundant sunshine and the various incentives available to homeowners, the association said. Growth of new installations has slowed for a variety of reasons, including aesthetic considerations, lack of understanding of the benefits of solar water heating, and the inability to finance the uppoint cost, according to the association.