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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Helena is G and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Saint Helena

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

Type G