power company vs public utility

power company and public utility both are nouns.

Word	Noun	Adjective	Verb	Adverb
power company	Yes	No	No	No
public utility	Yes	No	No	No

As nouns, public utility is a hypernym of power company; that is, public utility is a word with a broader meaning than power company:

power company: a public utility that provides electricity
public utility: a company that performs a public service; subject to government regulation

Other hypernyms of power company include public-service corporation, public utility company, utility.

power company (noun)	public utility (noun)
a public utility that provides electricity	a company that performs a public service; subject to government regulation