The timing could hardly be worse with South Korea in the grip of an extended heatwave and temperatures nudging 34 degrees Celsius (93F). One Seoul city government employee described her office as "one big dim-sum basket".
The heatwave is also smashing records in Japan, where the mercury hit 41 degrees Celsius today.
The coal-powered Dangjin III plant, with a capacity of 500 megawatts, was taken offline today by mechanical issues and will likely remain shut for a week, a spokesman for the state power distributor Korea Power Exchange (KPE) said.
The shutdowns come amid a lengthy disruption in South Korea's nuclear power sector.
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"We are facing potentially our worst power crisis," Trade, Industry and Energy Minister Yoon Sang-Jick said yesterday.
"We may have to carry out a rolling blackout... If one single power plant goes out of operation," Yoon said, appealing to factories, households and shops to curb consumption over the next three days.
The last time the Seoul government was forced to resort to nationwide load shedding was in September 2011, when unexpectedly high demand pushed power reserves to their lowest level in decades.
The resulting outcry forced the then-energy minister to resign.
The government has estimated that a future power outage on a similar scale would result in economic losses of around 11 trillion won (USD 10 billion).
National reserves of 4.0 gigawatts are considered the minimum necessary to guarantee a steady power supply.
If they drop below 2.0 gigawatts, it triggers an automatic alert requiring all government offices to turn off air conditioners, lights and any non-essential devices.
In a pre-emptive move today, the energy ministry ordered such measures effective immediately, even though the key reserve mark had not been breached.