Enea opened a 1,075 MW capacity unit built by Japan's Mitsubishi Hitachi Power Systems at its Kozienice plant to join a dozen other units in the 250-300 MW range at the site.
"The B11 section is the largest and most modern in Europe," said the firm's head of production Krzysztof Figat during a ceremony carried live over the internet to mark the 1.5 billion euro ($1.8 billion) project.
Total capacity at the plant, which uses some three million tons of coal annually, mainly from the Bogdanka mine in southern Poland, is now nearly 4,000 MW.
As well as the major plant at Kozienice, Poland is home to Europe's largest plant at Belchatow near the city of Lodz, one of the largest coal-fired plants in the world.
Coal remains a sector of primary importance in Poland, employing some 100,000 people.
Coal and lignite account for around 90 percent of Polish energy production.
"The new unit will increase energy security of Poland and the Polish people, which is an economic and political priority for our country," said new Prime Minister Mateusz Morawiecki.
Warsaw has recently slowed development of wind power and is still to decide on whether or not to build a first nuclear plant in a country suffering some of the worst air pollution in Europe.