A cash rebate for Hawaiian Electric Co. customers who install solar water heaters will be increased to $1,000 for a limited time in an effort to boost sales of the energy-efficient units.

The rebate, which had been $750, will be available for systems bought starting today, said Hawaii Energy, which administers the state’s energy conservation program in Honolulu, Maui and Hawaii counties. Hawaii Energy officials did not say how long the higher rebate would be available.

"If residents haven’t already done so, now is the time to take advantage of our increased rebate for solar water heating systems," said H. Ray Starling, Hawaii Energy program director.

Electric water heaters typically represent the single biggest charge on an electricity bill in households that don’t have air conditioning or a swimming pool, Starling said.

Households of four or more that switch from an electric to a solar water heater can save up to 40 percent, about $600 a year, on their electric bill.

Hawaii Energy’s $1,000 instant rebate, combined with state and federal tax credits, reduces the cost of the average solar water heating system from roughly $6,600 to about $2,000, according to the Hawaii Energy website.

For HECO customers who don’t want to pay for the cost of a solar water heater up front, Hawaii Energy offers an interest buy-down option.

Available in lieu of the instant rebate, Hawaii Energy provides a $1,000 incentive to participating lenders to buy down the interest rate on a qualifying solar water heating loan.

The lender then provides a zero to low interest loan depending on the applicant’s credit and other factors determined by the lender.

Residents can choose from a list of local banks and credit unions.