Aircraft electrical power can be derived from a variety of sources; these are categorized as either primary or secondary supplies. Batteries and generators are primary sources of electrical power; inverters and transformer rectifier units (TRUs) are secondary sources of power. This power is either in the form of direct or alternating current depending on system requirements. In addition to onboard equipment, most aircraft have the facility to be connected to an external power source during servicing or maintenance. The basic power source found on most aircraft is the battery, delivering direct current (DC).Generators can supply either direct or alternating current; the outputs of generators need to be regulated. Alternating current generators are also referred to as alternators.