South Korea's state power company issued a shortage warning on Monday, meaning that reserves are dangerously low, as electricity consumption rose sharply due to an unusual heatwave.
The warning from the Korea Electric Power Company (KEPCO) was aimed at averting power cuts, the knowledge economy ministry said, urging households, factories and other users to cut consumption voluntarily.
Temperatures have stayed above 35 degrees Celsius (95 F) for 10 consecutive days across the country, driving up air-conditioning use.
It was the first such warning since last September, when more than 2.1 million households and other premises were hit with rolling power cuts lasting up to one hour.
The ministry also resumed operations of the country's oldest nuclear power plant at Gori. It had been closed for months due to scrutiny over its safety and protests by civic groups.
"We are relieved to resume operations of the Gori reactor at a time when power consumption is expected to reach its peak," Knowledge Economy Minister Hong Suk-Woo said in a statement.
In February the Gori plant, built in 1978 near the southern city of Busan, briefly lost mains power and the emergency generator failed to kick in.
The incident did not result in any radioactive leaks but it sparked an extensive probe amid concerns over nuclear safety following last year's atomic crisis in Japan.
South Korea operates 23 nuclear power plants which meet more than 35 percent of its electricity needs.
Analysts say successive governments have failed to authorise major increases in the relatively low cost of electricity, encouraging wasteful consumption.
KEPCO last Friday decided to raise rates by 4.9 percent, yielding to government pressure to limit the increase to less than five percent.