American household electricity is 120 volts and 60 cycles per second, while European household electricity is 220 volts and 50 cycles per second. Electrical devices designed to use 110 volt electricity will draw almost twice the current when connected to 230 volt electricity, possibly causing severe damage to the device. Electrical devices with a built-in clock or timing function (electrical clocks, VCRs, etc.) will run slower by 12 minutes each hour when run on 50 cycles per second electricity. The shapes of the electrical plugs are also different between the USA, the British Isles and continental Europe, so plug adapters are also needed when traveling between these different areas.
There are two types of power adapters for reducing the European 230 volts to the American 120 volts. These are converters and transformers. Converters are small and light, but are not suitable for continuous use. They are limited to usage of about 30 minutes or so at a time. Transformers are larger and heavier, but are suitable for continuous use. Neither adapter type adjusts the electrical frequency, so any electrical timers will still not work accurately if used with a power adapter.
Converter-type power adapters are adequate for most travel needs, while transformer types are necessary for continuous duty applications. Also, be sure that the power rating of the power adapter is greater than the power requirement of the largest device that it will have to run, otherwise the power adapter may burn out.
Many devices have self-adjusting power supplies that can work on both American and European electricity. Examples are most laptop computers, cell phone chargers and camera battery chargers. Other devices, like hair dryers and irons, often have a small toggle switch that can be set to either 120 or 230 volts. A separate power adapter is not needed for such devices, although a plug adapter is usually still required.
