An existing 500-kilovolt transmission line will be extended from Bang Saphan district, Prachuap Khiri Khan, to Surat Thani and Phuket.
Authorities have been forced to throw the switch on the project because of the massive blackout that darkened all 14 southern provinces on May 21.
Phuket and virtually all the south was in darkness for as long as five hours. Samui has also suffered individual lengthy blackouts.
No details have been made public yet as to when the new extension will be in place, although 2019 has been mentioned.
Generators in the South cannot cope with demand so additional supplies are sent from the central region.
Thailand has a history of large-scale electricity failures.