power unit vs watt

power unit and watt both are nouns.

Word	Noun	Adjective	Verb	Adverb
power unit	Yes	No	No	No
watt	Yes	No	No	No

As nouns, watt is a hyponym of power unit; that is, watt is a word with a more specific, narrower meaning than power unit:

power unit: a measure of electric power
watt: a unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm

Other hyponyms of power unit include abwatt, milliwatt, W, kilowatt, kW, megawatt, H.P., horsepower, HP, var, volt-ampere, kilovolt-ampere.

power unit (noun)	watt (noun)
a measure of electric power	a unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm