On Tuesday, nationwide electrical demand was at 9,240 megawatts. Daily demand by early next week is expected to peak above 10,000 megawatts - higher than the all-time single-day record of 9,961 megawatts recorded Aug. 21, 2006.
Brief blackouts hit parts of Athens on Monday and the northern city of Thessaloniki on Tuesday, and power officials said further localized cuts would be unavoidable.
"We believe there will be some problems with the network - it is unavoidable is such conditions - but I don't think these will be widespread. Our maintenance crews are on hand to deal with problems quickly," Public Power Corporation spokesman Dimitris Lathouris said.
Increased use of air conditioners was being blamed for the extra pressure on the grid.
Temperatures in Athens were expected to reach 38 degrees Celsius (100 Fahrenheit) on Wednesday. By the weekend some parts of the mainland could reach 42 C (107.6 F), meteorologists said, peaking at 43 C (109.4 F) by Monday.
Meteorologists said the conditions were worsened by southerly winds from the Sahara.