charge per unit vs rate

charge per unit and rate both are nouns.

Word	Noun	Adjective	Verb	Adverb
charge per unit	Yes	No	No	No
rate	Yes	No	Yes	No

As nouns, charge per unit and rate are synonyms defined as:

charge per unit and rate: amount of a charge or payment relative to some basis

charge per unit (noun)	rate (noun)
amount of a charge or payment relative to some basis	the relative speed of progress or change
	amount of a charge or payment relative to some basis
	a quantity or amount or measure considered as a proportion of another quantity or amount or measure
	a magnitude or frequency relative to a time unit

charge per unit (verb)	rate (verb)
	assign a rank or rating to
	estimate the value of
	be worthy of or have a certain rating