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:

More specialised power supplies might be based upon:

  • Flywheels coupled to generators or alternators
  • Capacitors
  • Compulsators
  • Explosively pumped flux compression generators