The choice of your transformer depends on your requirements...

Anyone who is used to travel from UK to USA (or vice versa) knows that a simple matter may cause a big confusion. One of these matters is definitely electricity. What in UK works perfectly cannot be used in USA. The reason is that almost one half of the world is using 110 volts electricity whilst the other half is actually using 220 or 240 volts electricity. If you want to resolve permanently this problem (and the related ones) you need to buy a voltage transformer.

For those who are not used to these devices, here you can find some basic information about this electrical device. A voltage transformer is an electrical instrument permitting that an electrical appliance that has been designed to work using 110 volts electricity may be used in places where a different voltage (usually 220 or 240 volts) is normally used.

To give you an example, in UK electricity runs on 220 volts whilst in Japan it runs on 100 volts. Therefore, you could not use your electric shaver (if produced for the UK standards) when being in Japan unless you use a voltage transformer. The transformer simply will turn the electricity usually running on 100 volts up to 220 volts, allowing you to use your electric shaver.
At this point, how does a transformer work? In few words, the transformer converts the voltage of the electricity running to another voltage. The technical term to describe that is "step down/up". Actually, the market offers hundreds of transformers that allow to step up or down the electricity voltage feeding multiple electrical appliances as well. You need to ascertain the wattage of your appliance to calculate the size of your transformer. You can find the wattage description on a label aside of the appliance. Remember that if the appliance works by a motor more wattage will be required.

The choice of your transformer depends on your requirements, a step up or a step down or, even better, both functions. If not sure in relation to the transformer choice, my advice is to control the various products features with the manufacturer.