Hawaii Electric Light Co. has filed for a 6 percent rate increase for its Big Island customers next year.

In a filing with the state Public Utilities Commission, HELCO said the increase will help pay for the new generating unit at its Keähole power plant and two major transmission lines in West Hawai'i.

"These are tough times to be asking for a rate increase, but we have a responsibility to meet the current and future energy needs of our customers," HELCO president Jay Ignacio said in a statement.

The typical residential customer using 500 kilowatt hours a month will see the monthly bill increase by $9.57 to $177.51 under the new rates, the company said.

HELCO said the rate increase, which requires PUC approval, will raise $20.9 million a year.

That revenue will help pay for $289 million in capital investments made by HELCO during the past three years.

In June, HELCO put a new $92 million steam generating unit on line at its Keähole power plant.

The company said the new generator will not only help it meet the growing demands of West Hawai'i, which accounts for about half of the demand on the Big Island, but also help conserve fuel.

The new unit doesn't use oil as fuel.

It relies on steam produced as a byproduct of two other generators at the Keähole plant to create electricity.

If approved by the PUC, the rate increase would be HELCO's first since April 2007, when it received regulatory approval for a 7.58 percent rate hike.

"The rapid growth in West Hawai'i has shifted the demand for power on our island," Ignacio said.

HELCO also asked the PUC to allow it to adopt inverted tiered rates and optional time-of-use rates. These rates for residences would provide more opportunities for customers to manage their electricity bills.