The manufacture recommends you purchase a transformer that is 25% higher then the wattage you need.
You should know that you will never damage your appliance with a higher wattage transformer, however if you buy one that is not powerful enough, it will not work.
Remember to leave at least a 100% slack for the transformer. Using them to maximum capacity will eventually burn the transformer out after prolonged use. Some items, such as, laser printers and power tools spike when you turn them on. For these types of situations you need to buy a transformer that is at least 3 times the items capacity.
Step Down for overseas use only (TYPE A)
Step Up/Down for World Wide Use (TYPE B)
Deluxe Step Up/Down for World Wide Use (TYPE C)
BEST MODEL Step Up/Down for World Wide Use + Voltage regulator (Type D)
Step up/Step down for Japan to US and Vice Versa(Type E)