Maximum power adapter.

Any Apple charger which has a USB or USB-C output port compatible with the Apple magnetic charging cable will work. The external charger is simply a power supply (source of volts/amps). The rating on the label is its maximum capability, and the control of charging is inside your Apple Watch. The charging circuit will take as much power as it needs and any excess rating is simply ignored. Think of it like a high performance car, the engine can produce 600 horsepower but your right foot only requests about 50 hp when you drive through town.