Ptolemaida, former Kailaria, is a Greek town in the Kozani prefecture in the region of Macedonia. The city is best known for its coal mines of lignite and power station. It has been also called by Greek people "The Socket of Greece" because it has very strong power stations. Its population is 28.679 inhabitants according to the 2001 census. Ptolemaida is the biggest industrial, trade, and rural city of the prefecture and one of the most developed cities in industry in Greece.