Oahu residents will see their electrical bills rise slightly this month while customers on Maui will see continued volatility.

Hawaiian Electric Cos. said Monday that the typical HECO bill in October for an Oahu household using 500 kilowatt-hours was $139.91, or 25.9 cents a kilowatt-hour, up 17 cents from $139.74 in September when the effective rate was also 25.9 cents a kilowatt-hour.

“We’re seeing oil prices up slightly in recent months, but the outlook is generally stable,” HECO spokeswoman Shannon Tangonan said. “Regarding Oahu’s effective rate and the 17-cent difference, we’ve rounded the numbers so there will be slight variations in the bill amount even though the effective rates appear to be the same.”

Maui Electric Co. customers saw bills spike in October more than $9 after dropping about $7 in September. The average bill for a Maui household using 500 kilowatt-hours was $147.64, or 27.6 cents a kilowatt-­hour, compared with $138.24, or 25.7 cents a kilowatt-­hour, in September. In August the typical bill was $145.30.

“On Maui, at certain times of the year, wind resources can be more expensive than oil,” Tangonan said. “For this period, wind accounted for a larger percentage of generation. Also, fuel costs were higher.”

Fuel prices dictate electrical bills, as HECO’s energy mix is primarily made up of oil. In 2016 HECO’s energy mix was roughly 67.9 percent oil on Oahu. HECO imports oil primarily from Southeast Asia and the Middle East. The remainder of the energy comes from coal, 21.4 percent; solid waste, 6 percent; wind, 3.4 percent; solar, 0.8 percent; and biofuel, 0.5 percent.

The cost of electricity in the state is more than double the national average, as Hawaii residents paid an average of 29.25 cents a kilowatt-­hour for electricity in July. The national average was 13.12 cents in the same month, according to the latest figures from the U.S. Energy Information Administration.

Some electrical utility customers in Maui County saw bills increase from September.

MECO customers on Molokai using 400 kilowatt-­hours of electricity are paying an average of $136.35, or 31.7 cents a kilowatt-hour, up $6.36 from $129.99, or 30.1 cents a kilowatt-hour, in September.

Lanai residents using 400 kilowatt-hours of electricity are paying $146.72, or 34.3 cents a kilowatt-hour, up $1.28 from the $145.44, or 33.9 cents a kilowatt-hour, they paid in September.

Bills on Lanai and Molokai are calculated based on a typical household using 400 kilowatt-hours of electricity in a month. This is due to lower energy use compared with Maui, Hawaii island and Oahu, HECO said.

HECO, MECO and Hawaii Electric Light Co. monthly bills include the cost for kilowatt-­hours used, in addition to other charges.

The residential cost for HELCO customers’ October bill is $172.97, or 32.3 cents a kilowatt-hour, up $1.45 from $171.52, or 32.0 cents a kilowatt-­hour, in September.

The Kauai Island Utility Cooperative’s rate increased to 32.67 cents a kilowatt-­hour in October, up from 31.89 cents a kilowatt-­hour in September. The typical bill for a household using 500 kilowatt-­hours in October is $173.92, including a $10.58 customer base charge. In September the bill was $170.05.