An electricity bill at a mailbox in Seoul, Aug. 26. Yonhap

Extreme heat wave in August is forecast to push up electricity costs for households by an average of 13 percent, the state-run power company said Monday.

The average usage of electricity per household climbed 9 percent in August from a year earlier to come to 363 kilowatt-hours (kWh), which is likely to lead to a 13 percent on-year increase in the rates, or 7,520 won ($3.88) on average to 63,610 won, according to Korea Electric Power Corp (KEPCO).

The calculation is based on preliminary data, and the amount will be fixed at the end of this month, it added.

The sharper growth than the usage came as KEPCO currently applies the tiered rate system for residential power usage, where households using more electricity are charged at a higher rate.

Some 76 percent of total households in South Korea are expected to pay more this year than last year, KEPCO said.

The country's average daily maximum power demand reached a record high of 87.8 gigawatts in August by rising 6.1 percent on-year.

The number of heat wave days, or days when the daily high was 33 degrees Celsius or higher, reached 16 in August, the second-highest since 1973, when the data was first recorded.

The number of tropical nights, where nighttime temperatures remained above 25 C from 6:01 p.m. to 9 a.m. the following day, reached 11.3 days in August, marking the first time this figure has reached double digits, government data showed. (Yonhap)