Dictionary:

per annum

  (pər ăn'əm)

By the year; annually: a magazine subscription of 40 dollars per annum.

[Latin : per, per + annum, accusative of annus, year.]

Once each year.
Example: A Premium on the insurance policy is paid on a per annum basis. It amounts to $500 per annum.

i>-Lat: through the course of a year; annually. Anything (e.g., interest, wages, rent) which is calculated "per annum" is calculated on the basis of a year in time; sometimes a per annum rate will be fixed at 1/360th instead of 1/365th per day.

By the Year

The adverb has one meaning:

Meaning #1: by the year; every year (usually with reference to a sum of money paid or received)
  Synonyms: p.a., per year, each year, annually