Hanoi and 14 provinces in the northern and central regions have been recording temperatures of up to 38 degrees Celsius (100 degrees Fahrenheit), and the heatwave is likely to continue for another day or two, according to the National Center for Hydro - Meteorological Forecasting.
Some thermal power plants that have a combined output of 1,555 MW are reporting technical issues with turbines, according to national utility Vietnam Electricity (EVN).
Hanoi and some other localities recorded a power outage for around two hours on July 4 due to high demand.
EVN estimated earlier that the northern region would face a shortage of 1,500-2,400 MW during peak hours or extreme weather conditions this year.
The state utility has called on people to reduce power usage from 11 a.m. to 2.30 p.m. and form 8 p.m. to 10 p.m.
It also encouraged people to set their air-conditioner temperature at 27 degrees Celsius or higher and use it in combination with a fan.