Power Supply Wattage

A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity.

According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a personal computer are:

PC Item	Watts
Accelerated Graphics Port (AGP) card	20 to 30W
Peripheral Component Interconnect (PCI) card	5W
small computer system interface (SCSI) PCI card	20 to 25W
floppy disk drive	5W
network interface card	4W
50X CD-ROM drive	10 to 25W
RAM	10W per 128M
5200 RPM Integrated Drive Electronics (IDE) hard disk drive	5 to 11W
7200 RPM IDE hard disk drive	5 to 15W
Motherboard (without CPU or RAM)	20 to 30W
550 MHz Pentium III	30W
733 MHz Pentium III	23.5W
300 MHz Celeron	18W
600 MHz Athlon	45W

Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.