"The first 1,000 MW atomic power unit was synchronised with the grid between 6:30 a.m and 7 a.m. today (Tuesday). The power generation is around 300 MW," the official told IANS.
According to him, some tests have to be done as the unit was refuelled.
The power generation would be increased in a gradual manner, he added.
The first unit was shut down on April 13 for annual maintenance and refuelling.
Every year, one third of the reactor's 163 fuel assemblies, or 54 assemblies, are replaced.
Meanwhile, the second 1,000 MW unit that was shutdown on August 4 due to hydrogen concentration problem in stator is yet to restart.
India's atomic power plant operator Nuclear Power Corporation of India Ltd (NPCIL) has two 1,000 MW nuclear power plants at the KNPP built with Russian equipment.
Kudankulam is in Tirunelveli district around 650 km from Chennai.