What is the COP of the reversed Carnot cycle for refrigeration?

COP will be the quocient between what you want over what you payed for that is:

QC / WC

Or for a reversible carnot cycle since WC = QH-QC

QC / (QH-QC)

or

TC / (TH - TC)

Where

TC - Temperature of the cold reservoir

TH - Temperature of the hot reservoir

WC - Work of compression

QH - Heat given to the hot reservoir

QC - Heat taken from the cold reservoir