Hawaii Electric Light Co. has received approval from the state Public Utilities Commission for an interim rate increase of 7.58 percent for customers on the Big Island.

The increase, HELCO's first since early 2001, went into effect Thursday and is expected to generate $24.6 million in annual revenues.

The local utility said the average Big Island household using 500 kilowatt-hours a month will see its monthly bill increase by $11.41 to $163.42 per month from $152.01.

Warren Lee, HELCO's president, said the rate increase is not final. He said the PUC will hold hearings in May and issue a final order sometime after that.

If the PUC decides to grant a lower final rate increase, HELCO said, the difference will be refunded to customers with interest.

"It's still not finished yet. We still have to have evidentiary hearings in May and there has to be a final decision," Lee said.

HELCO said the majority of the interim increase will pay for transmission and distribution line improvements made over the last six years, as well as for two combustion turbine generating units that were added to HELCO's Keahole power plant in 2004.

The interim rate increase is part of a 9.2 percent, or $29.9 million, rate increase request submitted by HELCO last May.

In that filing, HELCO requested permission to implement a tiered rate structure designed to reward customers for energy conservation, with lower electric rates for lower kilowatt-hour usage tiers.

That rate design and other rate issues are still under review by the PUC and are not being implemented as part of the interim increase.

HELCO said its last base rate increase came in early 2001 when the PUC approved a 4.86 percent increase.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com.