The nuclear reactor's operator, TVO, said electricity output at the new Olkiluoto 3 (OL3) nuclear power reactor exceeded 1,000 megawatts on Friday.
OL3 is now Finland's largest electricity generation unit by capacity.
Teollisuuden Voima TVO said it expected the facility to reach full capacity—1,600 megawatts—in October.
On Thursday electricity operator Fingrid started two backup plants after OL3 produced less electricity than expected during trial operations. Similar startups of backup power plants occur quite often, several times a year, according to Fingrid.
What made the situation exceptional, however, was that regulated electricity was exceptionally expensive on Thursday morning, around 5,000 euros per megawatt hour.
Last month electricity output at OL3 dropped to zero due to a turbine failure during trial operations.
The Olkiluoto 3 project—which had initially been due to start full power electricity production in 2009—has been besieged by problems and setbacks, including faulty components, safety tests, and even a lawsuit.
The 1.6-gigawatt OL3 will become Finland's fifth and biggest reactor, and is expected to produce around 14 percent of the country's electricity once it is fully operational, possibly by the end of this year.
Olkiluoto will provide the Finnish grid with much needed supply, as Finland has said it may resort to rolling blackouts this winter.