surplus vs supererogatory

surplus is a noun but supererogatory is not a noun.

Word	Noun	Adjective	Verb	Adverb
surplus	Yes	Yes	No	No
supererogatory	No	Yes	No	No

As adjectives, surplus and supererogatory are synonyms defined as:

Other synonyms of surplus include excess, extra, redundant, spare, superfluous, supernumerary.

surplus (noun)	supererogatory (noun)
a quantity much larger than is needed

surplus (adjective)	supererogatory (adjective)
more than is needed, desired, or required	more than is needed, desired, or required