When travelling from the United States to the United Kingdom, travellers must take into account the fact that UK power outlets are different than US power outlets. Not only are the physical prongs different, but the frequency and voltage are not the same. Many tools and appliances will break if a voltage converter is not used, and others such as alarm clocks will not work correctly when plugged in. Read the label on the tool's power cord or user manual. If the voltage rating reads 100V~240V, then you do not need to use a voltage converter.
Multiply your appliance's voltage rating by the amperage that it draws. This will give you the power rating for your voltage transformer. For example, if the voltage rating is 110V and the amperage is 2 amps, the power rating is 220 watts.
Select a voltage transformer that is rated higher than your power requirements. This will prevent your transformer from burning out.
Select a grounded three prong plug adaptor. Many universal travel adaptors have grounded plugs. Make sure that it accepts grounded US plugs as it is unsafe to use power tools that are not grounded.
Plug your device into the converter, plug the converter into the plug adaptor, and plug the adaptor into the power outlet. Use your power tool as you normally would in the US.
Unplug the converter from the power outlet when you are finished using the tool to avoid burning out the converter.
If your power requirements are exceedingly high, it may be more economical to purchase the tool you are using in the UK rather than purchase an expensive voltage converter.
Never try to plug a grounded power cord into an ungrounded adaptor. Do not plug in a power tool that is not rated for 220V-240V into a power outlet in the UK without a voltage converter.