"We can confirm that unit 1 at Torness Power Station synchronised to the grid at 1232 (1132 GMT) today," a spokeswoman said.
The reactor stopped unexpectedly last week after an electric inverter failure which produced smoke.
EDF Energy's daily plant outage data showed on Friday the unit would return to service on Thursday, but it restarted three days early on Monday.
The utility stopped its 550-MW Dungeness B22 nuclear reactor on Friday for refuelling.
Copyright Reuters, 2010