do a power of good, to

do someone a power of good

 and do someone a heap of good
Rur. to be very good for someone. You should take a vacation. It'd do you a power of good. Just hearing your voice does me a heap of good.
See also: good, of, power

do a power of good, to

To effect a great improvement. This somewhat unsophisticated turn of phrase uses “power” in the sense of “much” or “a great deal.” For example, “Those vitamins will do her a power of good.”
See also: of, power