You Do Need power plug adapter while traveling from Saint Helena to Saint Kitts and Nevis.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Kitts and Nevis are D and G 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

Saint Kitts and Nevis

In Saint Kitts and Nevis, outlets of type D and G are in use.

Type D

Type G