Lower fuel prices and an amended supply contract whittled down Oahu customers’ electric bills this month.

Bills for residential households on the state’s most populous island fell in May for the eighth straight month, according to Hawaiian Electric Co., the state’s largest utility provider with service to Oahu, Maui and Hawaii island.

The bill for a typical household using 500 kilowatt-hours on Oahu is $140.48 this month, HECO said. The price per kilowatt-hour dropped to 26 cents this month from 26.1 cents in April when the average bill was $140.96.

The reduced bills are due to the falling price of fuel and an adjusted supply contract with Chevron, HECO spokesman Darren Pai said.

"Oahu customers are seeing the benefit of lower overall fuel prices and our recently completed efforts to secure lower pricing via an amendment to our fuel supply contract with Chevron," Pai said. "The full savings will be passed on to Oahu electric customers."

HECO estimates the amended contract will save customers approximately $22 million a year, or about $1.79 a month for a typical 500 kilowatt-hour residential bill. Chevron supplies 100 percent of the fuel for HECO’s plants on Oahu.

Pai said the amended contract will help balance out the costs associated with Oahu’s coal plant being out of service for part of April.

"In May, this reduction offset an increase in the cost of energy purchased from independent producers, helping to keep Oahu electric bills on their downward trend," he said. "Our total cost for purchased power in May is expected to be higher than in April because AES was going through maintenance part of last month."

HECO now reports the typical customer as one who uses 500 kilowatt-hours a month, down from its previous definition of "typical" as 600 kilowatt-hours due to a decline in customer energy use.

Hawaii has the highest electrical rates in the nation due in part to the state’s dependence on imported oil for most of its power.

In 2013, oil made up roughly 72 percent of the HECO companies’ energy mix.

Other states use natural gas, hydro­electric power or nuclear power to keep costs low.

The latest figures provided by the U.S. Energy Information Administration show that Hawaii’s price of electricity is more than double the national average. Hawaii residents paid an average of 30.85 cents a kilowatt-hour in February, while the national average was 12.29 cents.

Although Oahu prices decreased this month, some HECO customers on neighbor islands are seeing a slight bump on their May bills.

Hawaii Electric Light Co.’s residential rate on Hawaii island increased to 32.7 cents a kilowatt-hour from 31.7 cents in April when the average bill was $170.37. The May bill for a household using 500 kilowatt-hours of electricity is $175.10.

Maui Electric Co. customers using 500 kilowatt-hours of electricity will see an increase in rates on their bill to 29.5 cents per kilowatt-hour from 28.3 cents in April when the average bill was $151.30. Maui customers will pay an average of $157.06.

The Kauai Island Utility Cooperative lowered its rate to 32.3 cents a kilowatt-hour in May from 33.1 cents in April. The bill for a household using 500 kilowatt-hours of electricity, including a $10.58 customer base charge, is $172.08, down from $176.08.

Average monthly usage varies by island, but the 500-kilowatt-hour number is used to make comparisons. Monthly bills include the cost for kilowatt-hours used plus various other charges.

The typical customer bill on Lanai and Molokai is measured with electrical use at 400 kilowatt-hours due to lower energy use, HECO said.

Maui Electric Co. customers on Molokai using 400 kilowatt-hours of electricity will pay an average of $147.49 with effective rates of 34.4 cents per kilowatt-hour, a 1.1 percent decrease from 34.8 cents per kilowatt-hour in April when the typical bill was $148.81.

Customers on Lanai using 400 kilowatt-hours of electricity will pay the highest rates in the state at 35 cents per kilowatt-hour, a drop from the 37.6 cents per kilowatt-hour in April. Lanai residents will pay $149.64 in May compared with $160.22 in April.