Originally Posted by
bocastephen
Watts, I am not sure - I just need to step down USA household current to match Japan household current.
In this use case you will need a transformer that can handle the wattage of the appliance you're plugging into it. Overloading a transformer will just burn it out and require a new one.
Look at the appliance label where it shows things like the model number. There will be a wattage rating. What is the appliance?