MAXIMUMPC JUNE 2004
POWER SUPPLIES
Power Consumption Field Guide
Want to make sure your PSU generates enough power for your purposes? Put that calculator-watch to work
and find out how much power you really needCOMPONENT POWER REQUIREMENT LINE(S) USED
Older CPU 20 ~ 40 watts +3.3V
Athlon XP, Athlon 64, or P4 CPU 60 ~ 90 watts +12V
Motherboard 20 ~ 30 watts +3.3V, +5V, and +12V
RAM 20 watts per 256MB +3.3V
PCI card 5 ~ 10 watts +5V
AGP videocard 20 ~ 50 watts +5V and/or +12V
(3.3V from motherboard)CPU case fan 2 ~ 4 watts +12V
5,400rpm hard drive 5 ~ 10 watts +5V and +12V
7,200rpm hard drive 5 ~ 15 watts +5V and +12V
10,000rpm hard drive 5 ~ 20 watts +5V and +12V
Floppy drive 5 watts +5V and +12V
CD/DVD drive 10 ~ 20 watts +5V and +12V
Cold cathode lights 3 watts +12V
LED lights < 1 watt +12V or +5V
The following chart is a rough reference
for the power consumption of various
components found in a PC.
Remember, these are broad esti-
mates. Whenever possible you should
refer to the specification sheet for each
of your components; hard drives and
optical drives usually have accurate
figures on the drive labels themselves.
Most list the current rating in amps (A)
beside both 12V and 5V. For example,
a 7,200rpm Seagate Barracuda 160GB
hard drive has +5V 0.72A and +12V
0.35A printed on its label. Whenever
you get a voltage and current spec, you
may simply multiply them to get thepower requirement. This means that
the Seagate hard drive uses a total of
3.6 watts of +5V power (5 x .72) and 4.2
watts of +12V power (12 x .35).
For videocards, the unscientific rule
is that any card with a supplemental
12V power connector (such as the ATI
Radeon 9800) is likely a power hog. For
such high-performance cards, it’s wise to
assume they need 40 to 50 watts. Assign
milder videocards at least 20 watts.
As you can see, a performance PC’s
mobo, CPU, and graphics card hog
around 150 watts of power all by them-
selves. Much of this comes off the +12
volt line. A typical 300 watt ATX 12VPSU is rated to supply around 180 watts
of 12 volt power when working hardest.
This is before we allow for heat-deg-
radation in the PSU’s ratings, for other
components to also draw on the 12V
output, and for any headroom for future
growth. This is why we recommend you
use at least a 350W PSU. In fact, we
recommend you save yourself at least
30 percent of headroom between the
PSU’s rating and your maximum power
requirement. Pay special attention to
the total +12V output as many high-
power components in a contemporary
PC rely on this output. n