Hey there Welshy2001!
You don't need to worry about buying a charger that's too powerful - your portable battery is smart enough to "talk" to the charger to negotiate power
🙂.
If you're looking for a charger, you can use the one that came with your S20 - there should also be a Type-C to Type-C cable in your S20's box.
Hope this helps
Tom
🙂