Oahu’s residential electricity rates were higher in February and March largely because a coal-fired plant that provides the island’s cheapest source of power was down for maintenance, Hawaiian Electric Co. officials said Monday.

The 180-megawatt plant operated by independent power producer AES Corp. has the capacity to provide about 15 percent of the island’s peak electricity demand. The cost to produce electricity at the AES plant is about 9 cents a kilowatt-hour versus about 21 cents a kilowatt-hour for power produced by the oil-burning plants that provide the bulk of HECO’s electricity generation, according to a presentation HECO officials made to state legislators in January.

The typical bill for an Oahu household using 600 kilowatt-hours of electricity a month will rise to $218.75 in March from $216.11 in February, according to HECO. Both were significantly higher than the $200.91 typical bill in January.

HECO is charging 34.9 cents a kilowatt-hour for electricity this month, up from 34.5 cents a kilowatt-hour in February.

"Oahu bills are particularly high over these two months because AES, which burns less expensive coal, has been off-line for scheduled maintenance," said Peter Rosegg, HECO spokes­man. The AES plant was back in service earlier this month, Rosegg said.

Electric rates also will rise this month from February in Maui, Hawaii and Kauai counties.

Maui Electric Co. customers will see their rate rise to 37.0 cents per kilowatt-hour this month from 36.7 cents in February. The typical Maui bill will go up $1.49 to $230.72.

Hawaii island’s residential rate will rise to 39.7 cents a kilowatt-hour from last month’s 39.3 cents. The typical bill will go up to $248.59 from $246.14.

On Kauai the rate rose to 44.8 cents per kilowatt-hour from 43.2 cents in February, according to the Kauai Island Utility Cooperative.

Hawaii has the highest electrical rates in the nation. The statewide average of 36.3 cents a kilowatt-hour in December was triple the national average of 11.6 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 8.4 cents a kilowatt-hour.