Rising energy costs and environmental concerns have raised awareness of
opportunities to conserve and reduce electricity used by Information Technology equipment,
especially computers. Computer power management software and power supply efficiency are key
elements in the drive to conserve energy resources. Before examining these elements let's go
over a basic primer on power supplies and the computers they run.
Power is measured in Watts (W), Voltage in Volts (V) and Amperage in Amps (A).
Power = Volts x Amps or P = V·I. Power supplies are rated by maximum power output,
the sum of each voltage-current multiplication.
A computer power supply outputs several steady DC voltages. Each voltage has a
minimum and maximum allowable current range. Computer devices draw variable current from one or
more of the steady DC voltages. Since Power = Voltage x Current, and voltage is always constant,
it's easy to see power usage is highly dependent on the computer's current draw. Also, another basic
point is the power supply must always be able to supply the maximum current required. The
computer should use power only when necessary. The power supply should deliver the power in an
AC stands for Alternating Current. AC is delivered over transmission lines by
power utilities and has a sinusoidal shape. DC stands for Direct Current. It is linear. All
digital circuits require DC power.
The role of power management software is to determine when a computer device, such
as a hard drive, is doing useful work or not. If the device is detected as inactive it can be
placed in a low power standby or sleep state, substantially lowering or eliminating current draw.
This software also provides the ability to create power schemes, trading high system performance
for lower power consumption. All major operating systems, such as Microsoft XP, have built in power
management software based on Intel's ACPI (Advanced Configuration and Power Interface) specification. ACPI
allows operating system control of device power states.
The previous power management technology was APM (Advanced Power Management), also
by Intel. APM was implemented in BIOS and delivered mixed results.
Computer power supply efficiency is a more direct, measurable, and tangible target
for conservation. Power supplies are energy conversion devices. They convert the AC input into DC
outputs required by computer circuitry and peripherals. During this conversion, a certain amount of
energy is lost as heat. This heat loss determines power supply efficiency. Typical efficiency is
70%, in general very good by energy conversion standards. It is worth noting computer power
supplies are a class known as switching power supplies, inherently more robust and efficient, than
their linear power supply counterparts.
Efficiency is calculated by dividing Power Out by Power In.
Efficiency = Pout / Pin
However, there is room for improvement and given the vast number of computer
power supplies in service, the savings could be huge. It is estimated office computers consume over
320kwh (kiloWatt Hours) per year. Improving efficiency to 80% results in savings of approximately
45kwh per year. Electric utilities are eager to capture this savings and have formed the
. 80PLUS aims to integrate more
energy efficient power supplies into desktop and server computers. The
computer specification now includes 80PLUS power supply efficiency rating. 80PLUS has gained wide
acceptance with leading power supply manufacturers, OEMs, and consumers.