You Do Need power plug adapter while traveling from Channel Islands (Guernsey & Jersey) to San Marino.

Here is Why You Need An Adapter

The outlet and plug type used in San Marino are C, F and L and in Channel Islands (Guernsey & Jersey) are C and G. This is how they look like.

Channel Islands (Guernsey & Jersey)

In Channel Islands (Guernsey & Jersey), outlets of type C and G are in use.

Type C

Type G

San Marino

In San Marino, outlets of type C, F and L are in use.

Type C

Type F

Type L