You Do Need power plug adapter while traveling from Namibia to Solomon Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Solomon Islands are G and I 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

Solomon Islands

In Solomon Islands, outlets of type G and I are in use.

Type G

Type I