rate vs charge per unit

rate and charge per unit both are nouns.

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

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

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

rate (noun)	charge per unit (noun)
the relative speed of progress or change	amount of a charge or payment relative to some basis
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

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