WARSAW, Poland (AP) — Energy experts were working Tuesday to restore full operations at Poland’s biggest power plant, the lignite coal-fueled Belchatow, after an energy network failure switched off 10 of the plant’s 11 units.

Energy imports from Germany and Sweden were needed to fill in for the suddenly missing power. Belchatow provides up to 20% of Poland’s energy.

The outage occurred late Monday, due to a failure of power station where 10 of Belchatow's units are connected. The largest, 11th unit, is connected to a different power station and was not affected.

Poland's chief energy distributor, PGE, said early Tuesday that work had been restored in six of the units, but that restarting them after a total cut was a big challenge.

Belchatow, in central Poland, combines Europe's largest lignite mine and Poland's largest power plant. With total power of some 5.3 GW, it supplies energy to some 11.5 million households.