The Gas Co., Hawai'i's leading gas energy provider, has filed for an 8.4 percent rate increase that would affect primarily residential customers.

The request was filed with the state Public Utilities Commission and would affect about 35,000 of The Gas Co.'s 67,000 customers statewide. The new rates would apply to customers who are served by underground pipeline, and would not include tank and bottled gas customers, the company said.

The Gas Co. said this is its first request for a rate adjustment in eight years and would generate about $12.5 million for the company — $8.4 million from commercial customers and $4.1 million from residential customers. Officials said the increase is necessary to pay for higher operational and maintenance costs, capital investments, renewable energy projects and compliance with new federal regulations.

If approved, the average residential monthly bill of $70 would go up by about $10.62, or 35 cents a day, the company said. About 90 percent of the 35,000 customers affected by the hike are residential.

The 8.4 percent rate increase, which is an average for all customers, would appear on bills beginning in mid-2009.

The proposed rate hike is separate from increases related to fuel costs, which are passed on to customers in their monthly bill.

In July, O'ahu customers saw a fuel adjustment of $3.84 per therm, which is the billing unit, and $1.50 per therm on the Neighbor Islands.

The average customer consumes about 12.4 therms a month, the company said.

Jeffrey Kissel, The Gas Co. president and chief executive officer, said increasing costs have forced the company to seek a rate increase.

"With the rising cost pressures of doing business in Hawai'i and the importance of addressing Hawai'i's energy issues, we can no longer defer our request for a rate increase," Kissel said in a statement.

The Gas Co. has been Hawai'i's only full-service gas energy company for more than 100 years.

The company has more than 300 employees and 1,000 miles of gas lines statewide and provides gas energy and related services to 67,000 commercial and residential customers on O'ahu, the Big Island, Maui, Kaua'i, Moloka'i and Lana'i.