A Power Supply Unit or Switch Mode Power Supply(SMPS) converts the A.C. to lower D.C. voltage for use of internal components of the computer.
Internal components of computer require steady low voltage direct current (D.C) for normal operations. While most of the power from power sockets is in alternate current (A.C), a unit is required to convert the alternate current to direct current. Power Supply Unit does the job of converting this A.C. Current to D.C. current.
Power Supply is rated by the number of watts it produces. The Power range of typical power supply unit from 250 watts to 1000 watts for a high-performance gaming computer with multiple graphics cards. A typical home desktop usually requires 300 – 500 watts.
If a power supply output is less than the power requirement of the computer, the computer may freeze, crash frequently. So care should be taken when deciding on power supply unit for your computer.