Measuring Your Power Struggles

How much power do you need? If you're running a micro-ATX motherboard with embedded graphics and audio, and a single 5400rpm IDE drive, that whimpy 150W power supply may be enough. But if your rig pushes half a gig of RAM, dual procs, Ultra2 SCSI RAID, and a GeForce 256, even a 300W supply may leave you bent.

Don't judge your power supply by overall wattage alone, look at the demands on each separate line. Nonetheless, when looking at overall wattage, add the requirements of every device in your rig, then multiply that number by 1.5 (because you should be taxing from 30 precent to 70 pecent of your power supply's capacity). Some of your individual components may actually list their specific power requirements, so use these numbers for more accurate calculations. And remember, if you need to figure out wattage, multiply the voltage by the amps.

  Line(s) Used
High-wattage AGP card 20 - 50W +3.3V
Average PCI card 5W +5V
Cached SCSI controller PCI card 20-25W +3.3V and +5V
Floppy drive 5W +5V
10/100 NIC 4W +3.3V
50x Atapi CD-ROM 10 - 25W +5V and +12V
12x Atapi DVD-ROM 10 - 25W +5V and +12V
12x / 10x / 40x SCSI CD-R/RW 20W +5V and +12V
SCSI CD-ROM 12W +5V and +12V
RAM 8W per 128MB +3.3V
Ultra2 SCSI PCI card 5W +3.3V and +5V
7200rpm IDE hard drive 5 - 15W +5V and +12V
7200rpm Ultra2 SCSI hard drive 24W +5V and +12V
10,000rpm SCSI drive 10 - 40W +5V and +12V
Motherboard (without CPU or RAM) 20 - 30W +3.3V and +5V
Typical Pentium II 30W +5V
1GHz Pentium III 34W +5V
1.7GHz Pentium 4 65W +12V
1.4GHz AMD Athlon 70W +12V


Multiples of bytes
Decimal prefixes Binary prefixes
Name Symbol Multiple Name Symbol Multiple
kilobyte kB 103 kibibyte KiB 210
megabyte MB 106 mebibyte MiB 220
gigabyte GB 109 gibibyte GiB 230
terabyte TB 1012 tebibyte TiB 240
petabyte PB 1015 pebibyte PiB 250
exabyte EB 1018 exbibyte EiB 260
zettabyte ZB 1021
yottabyte YB 1024

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