Q. What is Switched Mode Power Supply?

SMPS is the unit into which electric supply from the mains is connected to your PC and this supplies DC to internal circuits. It is more efficient, more complex and less expensive than linear supplies.

SMPS performs in the following way:  The electric supply received is sent to a component known astriac which shifts it from 50 Hz to a much higher frequency (approximately 20.000 Hz). At the same time, employing a technique known as Pulse Width modulation, the pulse is varied to the requirements of the computer circuit. Shorter pulses provide lower output voltage. A transformer then decreases back the voltage to the correct levels and rectifiers and filters produce the pure DC current.

SMPS has two main benefits: They produces less heat since they waste less power and use less expensive transformers and circuits because they operate at higher frequencies. 

The power requirement of a PC relies on peripherals and motherboard in your computer.  Still in modern PCs, your requirement may not be more than 150-200 Watts.