You Do Need power plug adapter while traveling from Saint Helena to Cuba.

Here is Why You Need An Adapter

The outlet and plug type used in Cuba are A, B, C and L and in Saint Helena is G. This is how they look like.

Saint Helena

In Saint Helena, outlets of type G are in use.

Type G

Cuba

In Cuba, outlets of type A, B, C and L are in use.

Type A

Type B

Type C

Type L