A power supply unit (sometimes abbreviated power supply or PSU) is a device that supplies electrical power to a device or group of devices. The term is most commonly applied to units that are integrated with the devices they supply, such as computers and household electronics, and never to devices supplying, conditioning or otherwise supporting an electric utility grid. (For large-scale power supplies, see electricity generation.)
The range of different types of power supply is very broad, since widely differing design criteria affect each application.
    As well as the usual requirements of cost, reliability, weight and size, constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it for without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.
Common power supply technologies include: 
• Batteries
• Chemical fuel cells
• ...and other forms of power storage systems
• Solar power
• Conversion of another form of electrical power into the desired form (typically converting 120 or 240 volt alternating current supplied by a utility company (see electricity generation) into low-voltage direct current for electronic devices); see switched-mode power supply, linear regulator, rectifier, inverter (electrical)
• Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has rotational power to spare, or in semi-portable units containing an internal combustion engine and a generator)