Residential electricity rates will decline slightly on O'ahu and Maui this month, while rising on the Big Island and Kaua'i.

Hawaiian Electric Co. said the typical 600-kilowatt-hour residential bill on O'ahu will decline to $117.45 from $117.94 in April.

The effective rate for electricity in Honolulu will drop to 18.15 cents per kilowatt-hour from the 18.23 cents charged last month.

The O'ahu rate is the lowest since the summer of 2007.

Elsewhere in the state, electricity rates are mixed:

• Maui customers will see their rates drop to 21.88 cents from 22.01 cents in April.

• Big Island residential rates will rise to 29.07 cents per kilowatt-hour from 28.82 cents.

• Kaua'i Island Utility Cooperative's May rate is 25.43 cents, up from 22.37 cents last month.

Rates spiked in Hawai'i last year because most of the electricity is produced using petroleum products such as fuel oil or diesel oil, which rose in cost. On O'ahu, rates peaked in September, when residential customers' monthly bill averaged $202.13.