Residential electricity rates fell on Oahu in September for the third consecutive month, mainly reflecting a decline in Hawaiian Electric Co.’s fuel costs.

HECO said a typical 600-kilowatt-hour bill for Oahu residential customers fell by $5.83 to $209.82 in September from $215.65 in August. The effective rate for electricity on Oahu is 33.50 cents a kilowatt-hour, down from 34.5 cents a kilowatt-hour last month. Electric rates also fell in August and July.

Elsewhere in the state:

» Maui Electric Co. customers saw rates fall to 34.9 cents per kilowatt-hour this month from 35.40 cents in August. The typical Maui bill fell by $3.18 to $218.12.

» Hawaii island residential rates rose to 40.7 cents a kilowatt-hour from last month’s 40.1 cents. The typical bill rose by $3.87 to $254.69.

» On Kauai the rate rose to 43.1 cents per kilowatt-hour. Last month the rate charged by the Kauai Island Utility Cooperative was 40 cents per kilowatt-hour.

HECO and KIUC adjust their rates monthly largely to reflect changes in fuel costs. The utilities generate about 80 percent of their electricity from petroleum, making rates sensitive to swings in oil prices.

KIUC officials said they hope to reduce fuel costs by adding less-expensive recycled oil to the fuel mix at one of their petroleum-fired power plants. The fuel would be mixed 50-50 with the diesel currently burned at KIUC’s Port Allen power plant, according to the utility.

The utility is seeking permission from the state Public Utilities Commission to buy at least 500,000 gallons a year from Kapolei-based Uni­tek Solvent Services. KIUC burns 30 million gallons of oil annually at its two plants.

Hawaii has the highest electrical rates in the nation. The statewide average of 39.9 cents a kilowatt-hour in June was more than triple the national average of 12.1 cents a kilowatt-hour, according to the most recent numbers available from the U.S. Energy Information Administration. Washington state had the lowest average rate at 8.7 cents a kilowatt-hour.