Don't rely on the computer's operating consumption alone, which can be obtained by adding the requirements for the devices in the PC. Remember that a much larger drain occurs as the machine powers up, when hard drives and other heavy feeders simultaneously compete for the available startup power. Most general-use computers require 130 watts while running and about 200-205 watts when booting. Sound cards, modems, and (worst of all) monitors attached with an accessory plug in the case can push a weak power supply to its limit and beyond.
Servers and high-performance workstations often have an abundance of RAM, multiple drives, SCSI (Small Computer System Interface) cards, and power-hungry video adapters, along with one or more network cards. They often demand power supplies of 35-500 watts.
CAUTION
The label on a power supply that says "Don't Open" means just that! Opening a power supply is dangerous. It is better to completely remove and replace a defective power supply as needed.
Components power consumption
Motherboard: 50 to 150 watts
Processor: 80 to 140 watts
RAM: 15 watts per slot
Fan: 3 or 4 watts per fan
Hard Drive: 20 to 30 watts
SCSI: 20 watts minimum
CD/DVD ROM: 20 to 30 watts
PCI Card: 5 watts per slot
AGP: 30 watts minimum
PCI Express: 50 to 150 watts