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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Helena is G and in Equatorial Guinea are C and E. This is how they look like.

Equatorial Guinea

In Equatorial Guinea, outlets of type C and E are in use.

Type C

Type E

Saint Helena

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

Type G