Residential electricity rates fell in August on all islands except for Kauai.

Hawaiian Electric Co. said a typical 600-kilowatt-hour bill for Oahu residential customers fell by $1.20 to $215.65 in August from $216.85 in July. The effective rate for electricity in Honolulu declined to 34.5 cents a kilowatt-hour from 34.7 cents a kilowatt-hour.

Elsewhere in the state:

» Maui Electric Co. customers saw rates fall to 35.4 cents per kilowatt-hour this month from 37.5 cents in July. The typical Maui bill fell by $12.26 to $221.30.

» Hawaii island residential rates fell to 40.1 cents a kilowatt-hour from last month’s 41.4 cents. The typical bill fell by $7.86 to $250.82.

» On Kauai, the rate rose to 40 cents per kilowatt-hour. Last month the rate charged by the Kauai Island Utility Cooperative was 38.4 cents per kilowatt-hour.

HECO and KIUC adjust their rates monthly largely to reflect changes in fuel costs. The utilities generate 75 percent of their electricity from petroleum, making them vulnerable to swings in oil prices.

Hawaii has the highest electric rates in the nation. The statewide average of 37.67 cents a kilowatt-hour in May was more than triple the national average of 11.97 cents a kilowatt-hour, according to the most recent numbers available from the U.S. Energy Information Administration. Idaho had the lowest average rate at 8.24 cents a kilowatt-hour.

Hawaii’s high exposure to volatile oil prices was one of the main factors behind the passage of the 2008 Hawai’i Clean Energy Initiative, which called for the state to use energy efficiency and renewable resources to supply 70 percent of energy needs by 2030.