You Do Need power plug adapter while traveling from Portugal to Tuvalu.

Here is Why You Need An Adapter

The outlet and plug type used in Tuvalu is I and in Portugal are C and F. This is how they look like.

Portugal

In Portugal, outlets of type C and F are in use.

Type C

Type F

Tuvalu

In Tuvalu, outlets of type I are in use.

Type I