How do I know what wattage power supply I need?

The internal components of your PC determine what wattage power supply you need. Below is a list of common components and their watt usage. This is only a guide with all wattages estimated.

• ATX Motherboard - 30W-40W
• AMD Athlon 900MHz-1.1GHz - 50W
• AMD Athlon 1.2GHz-1.4GHz - 55-65W
• Intel Pentium III 800MHz-1.26GHz - 30W
• Intel Celeron 700MHz-900MHz - 25W
• Intel Celeron 1.0GHz-1.1GHz - 35W
• Intel Pentium 4 1.4GHz-1.7GHz - 65W
• Intel Pentium 4 1.8GHz-2.0GHz - 75W
• 128MB RAM - 10W
• 256MB RAM - 20W
• IDE CD-ROM Drive - 20W
• IDE DVD-ROM Drive - 20W
• SCSI CD-RW Drive - 17W
• SCSI CD-ROM Drive - 12W
• 5400RPM IDE Hard Drive - 10W
• 7200RPM IDE Hard Drive - 13W
• 7200RPM SCSI Hard Drive - 24W
• 10000RPM SCSI Hard Drive - 30W
• AGP video card - 20-30W
• PCI video card - 20W
• Floppy Drive - 5W
• Network Card - 4W
• Modem - 5W
• Sound Card - 5W
• SCSI Controller Card - 20W
• Firewire/USB Controller Card - 10W
• Case Fan - 3W
• LED Case Fan - 6W
• CPU Fan - 3W

We have also come across this website which will help you calculate the amount of wattage your PC will use.