Every new laptop computer comes with an AC power adapter that converts power from an AC wall outlet to the DC power required to run the computer and charge the internal battery. The input voltage and the type of AC connector or power cable must be correct for the domestic AC supply in the country where you expect to use the computer.
When local AC power is available, you should use the power adapter to run the computer. If you're paying for it, AC is considerably cheaper than battery power, and the supply doesn't run down after a few hours.
If you use the same computer in one or more semi-permanent locations, it can be convenient to buy extra power adapters. This allows you to keep one adapter plugged into the wall behind your desk (or wherever you use the laptop most often), and keep the other one in your computer bag for temporary use in the library, in a coffee shop, or a hotel room.
Retailers and Web sites that sell batteries and accessories for laptop computers often offer power adapters at lower prices than the ones sold by the computer's manufacturer. Before you buy one of these third-party adapters, be sure it is intended for your particular make and model. The output voltage and the cable connector should be exactly the same as the one that came with your computer, and it should be rated for at least the same number of watts and amps. Be sure the adapter carries the certification mark from Underwriters Laboratories (UL) or the comparable testing agency for your own country or region.
Before you try to use your computer's power adapter in a place that uses a different AC voltage from your home country, read the adapter's label carefully to be sure it is rated for the local power source. If it's not, or if the local AC power outlets at your destination use a different kind of plug, ask the manufacturer's technical support center or look on their Web site for information about ordering a different adapter or cable.
The alternative to a replacement cable or AC adapter is a simple AC plug adapter. These are widely available from airport shops, luggage stores, and other retailers who specialize in products for travelers.
When you plan to travel overseas with your laptop computer, take the time to find the adapter or cable needed at your destination before you leave home. Unless you can visit someplace like Singapore's Funan Mall (where you can probably find just about any kind of computer part), you could waste a lot of time in an unfamiliar city if you have to search for a computer part.