Residential electric rates fell on Oahu in September to their lowest level in three months largely because of a decline in fuel costs, Hawaiian Electric Co. reported.

September power rates rose on all other islands.

The bill for a typical household on Oahu using 600 kilowatt-hours of electricity a month is $203.06 in September, $7.08 lower than in August. The September rate of 32.3 cents a kilowatt-hour is the lowest since May when it was 31.8 cents a kilowatt-hour.

The decline in Oahu’s electric rate was primarily due to a drop in the price for low-sulfur fuel oil, which accounts for more than 50 percent of a customer’s bill, a HECO spokes­man said.

HECO paid $118.37 for a barrel of LSFO in September compared with $131.01 in August. Oahu is the only island where LSFO is burned for power generation. The other islands use a mix of diesel, medium-sulfur fuel oil and naptha.

The reliance on oil for power generation is one of the primary reasons electricity in Hawaii is nearly three times the national average. Hawaii residents paid an average of 36.6 cents a kilowatt-hour for electricity in July compared with the national average of 12.6 cents a kilowatt-hour, according to the latest data available from the U.S. Energy Information Administration. Washington state, which is one of the nation’s leaders in hydropower, had the lowest electric rate at 8.9 cents a kilowatt-hour.

Around the state, electrical rates were the highest on Kauai, where residents paid 42.3 cents a kilowatt-hour, up from 41.6 cents a kilowatt-hour last month.

Hawaii island’s residential rate rose to 40.4 cents a kilowatt-hour from last month’s 39.4 cents. The typical bill rose by $5.50 to $252.63.

Maui Electric Co. customers saw their rate increase to 36.3 cents per kilowatt-hour from 35.9 cents per kilowatt-hour last month. The typical Maui bill rose $2.40 to $226.523.

Hawaii’s high electricity prices have driven a surge in installation of solar photovoltaic systems. Hawaii ranks third nationally in total PV generating capacity after No. 1 Arizona and second-ranked Nevada.