You Do Need power plug adapter while traveling from El Salvador to Saint Lucia.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in El Salvador are A and B. This is how they look like.

El Salvador

In El Salvador, outlets of type A and B are in use.

Type A

Type B

Saint Lucia

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

Type G