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

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M 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

Namibia

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

Type D

Type M