Updated April 2016
The power supply in Bali and Jakarta, like in the rest of Indonesia where it is available (many rural areas still lack electricity), operates at 220 volts / 50 Hz. The UK and most Continental European countries, as well as Australia, supply voltage at 230 volts, although some like Denmark, Switzerland or Italy also use 220 volts. Some devices expecting 230 volts, like irons or kettles, may take a little longer to reach their maximum operating efficiency, but will work reasonably well.
North, Central and South America operate at 110-120 volts / 60 Hz (and Japan: 100 volts, and 50 Hz or 60 Hz, depending on the area). Their electrical appliances must not be used in Indonesia, as they will overheat and may cause a fire. However, most hotels, nowadays, provide hair-dryers, kettles, and irons on request; so you should leave yours at home.
Nowadays, most travelling appliances accept a wider range of voltage, from 110 volts to 240 volts, and are adaptable to most countries' supply. Check carefully the voltage stamped on the device before plugging it into the socket.
The wall sockets are generally of the European type: two round pin type, although some hotels have installed British 3-rectangular pins, or the Australian V shaped type with 3 pins.
If you are travelling from Australia, New Zealand, and other countries in the area, it is a good idea to take a travel adapter to enable you to use computers, or charge your mobile, without having to rely on the hotel to provide you with one. For those from areas where the supply is around 110 volts, an adapter is not enough; you would need a voltage converter, or maybe to buy locally a cheap replacement.
Portable travel voltage converters are designed to be used by small electronic gadgets, and in the case of heating devices for very short periods of time only (around 15 minutes) and rated below a certain voltage (around 1600v). Before you get one, make sure it will be useful and safe for the use you intend to give it.
If you are planning a longer trip and intend to stay at rural accommodation, bear in mind that the supply may be unstable and computers may need a voltage stabilizer.
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