External power supplies are used both with equipment with no other source of power and with  battery-powered equipment, where the supply, when plugged in, can sometimes charge the battery in addition to powering the equipment.

Use of an external power supply allows portability of equipment powered either by mains or battery without the added bulk of internal power components, and makes it unnecessary to produce equipment for use only with a specified power source; the same device can be powered from 120 VAC or 230 VAC mains, vehicle or aircraft battery by using a different adapter. Another advantage of these designs can be increased safety; as the hazardous 120 or 240 volt mains power is transformed to a lower safer voltage at the wall outlet, and the appliance which is handled by the user is powered by this lower voltage.

An AC adapter or AC/DC converter is a type of external power supply, often enclosed in a case similar to an AC plug. Other common names include plug-in adapter, power brick, and power adapter. Adapters for battery-powered equipment may be described as chargers or rechargers ;(see also battery charger. AC adapters are used with electrical devices that require power but do not contain internal components to derive the required voltage and power from mains power.