Intro to Computer Systems

Chapter 9: Power Considerations

Idle Power Requirements

Most of a computer's operating time is spent idle; thus this operation mode has been selected by system designers as a prime candidate for improving the power efficiency of computer systems.

Idle vs Active Power Requirements

A computer, by its very design, uses less power when idle anyway: not many transistors on the processors are switching, the hard drive may be spinning but not active, etc.

However, having the hardware in a state 'ready to work' is relatively inefficient: since it only takes microseconds for semiconductors to 'wake up', there are significant gains to be had in dynamic power management.

Dynamic Power Management

Dynamic power management refers to a power management regime that changes due to circumstances - actively finding opportunities for something to run slower, or to shut down, all to save power. Most of these techniques were first implemented on portable computers, where battery life means that power is a precious commodity; however they have since been folded into desktop computing in an attempt to reduce the cost of owning and operating a computer.


The CPU is a prime source of power savings. There are a number of design options available to lower power consumption when full performance is not necessary:

A diagnostic program (CPU-Z) shoing a 2.67 GHz processor underclocking itself to 2.17GHz to save power.
A diagnostic program (CPU-Z) shoing a 2.67 GHz processor underclocking itself to 2.17GHz to save power.
Other Devices

Other devices can have similar techniques used to lower power consumption when idle:

Power Efficiency Certification

The positive financial and environmental impacts of lowering computer power consumption have signaled the need for certification bodies, so that consumers of equipment can be guaranteed that hardware being purchased supports certain power-saving modes and will not waste power unnecessarily.

The Energy Star logo.
The Energy Star logo.

Energy Star

Energy Star is the most common power certification for technology products (not limited to just computer components). It originally concentrated on computer monitors and printers, two devices that consumed a lot of idle power, and the technology to save power was not particularly complex.

The Energy Star product certification process is controlled by the United States Department of Energy, which documents the requirements for energy-efficient products in various sectors.

The current Energy Star Specification for computers is at version 5.2.

It mandates minimum standards for power supply efficiency, various power-saving modes supported by the hardware, standby power usage, and typical energy consumption.


80 PLUS is a certification specific to computer power supplies. At its core, it mandates a minimum 80% energy efficiency at 20, 50 and 100% of maximum load. This spurred development in energy-efficient power supply design, and the standard has since been updated with higher level certifications with greater efficiency requirements.

80 PLUS efficiency requirements.
80 PLUS efficiency requirements.

One shortcoming of the 80 PLUS certification requirements is that it does not specify a minimum efficiency level for a very low load. Thus, using an 80 PLUS certified power supply is not a substitute for sensible component selection.