You Do Need power plug adapter while traveling from Saint Helena to Antigua and Barbuda.

Here is Why You Need An Adapter

The outlet and plug type used in Antigua and Barbuda are A and B 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

Antigua and Barbuda

In Antigua and Barbuda, outlets of type A and B are in use.

Type A

Type B