fezzzer
macrumors 6502
LOL, knew that was coming. Even bearded guys need to clean up their necks. But then there's the ladies. Goops... 😉
Apple says 18w or greater, and it needs to be PD-capable. I sort of doubt that there are too many USB-C adapters of 18w or more that aren't PD-capable, but you should make sure that it is if you want fast charging.
About fast charge on Apple Watch - Apple Support
Learn about Apple Watch models that support fast charge, and how to make sure your watch charges as quickly as possible.
Apple's support article suggests a USB-C power charger of 5W and greater will work provided it supports USB-PD.