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:
- Accelerated Graphics Port (AGP) card = 20 to 30W
- Peripheral Component Interconnect (PCI) card = 5W
- small computer system interface (SCSI) PCI card = 20 to 25W
- 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 = 5-15W
- 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.