Computer power management

What's the big deal?  

Research shows that personal computers (PC) are not being actively used during the vast majority of the time that they are kept on.  It is estimated that an average PC is in use 4 hours each work day and idle for another 5.5 hours.  It's also estimated that some 30-40 percent of the US's work PCs are left running at night and on weekends.

Office equipment is the fastest growing electricity load in the commercial sector.  Computer systems are believed to account for 10 percent or more of commercial electricity consumption already.  Since computer systems generate waste heat, they also increase the amount of electricity necessary to cool office spaces.  (Yes, they lower the cost of heating somewhat.  That's not a big factor in Miami.)

For the Medical Center, we estimate the savings from PC power management to be hundreds of thousands of dollars annually, even without factoring in increased office cooling costs.  Considerable savings are also possible from easing wear-and-tear on the computers themselves.

If you'd like to make a savings calculation for yourself or your organization -- on electricity, dollars, trees, CO2 emissions -- you can do that here.

Isn't this "automatic" on most computers? 

Almost all computers and monitors sold in the US today come with ENERGY-STAR energy-saving features.  But they generally don't work unless you set them. 

Both Windows and Apple/Mac systems allow you to set the amount of idle time that occurs before the system goes into "standby" or "sleep"  mode:

• On Microsoft (Windows) systems, times are set in the Power Options section of the Control Panel.  Get there by the following path: Start > [My Computer >] Control Panel > [Performance and Maintenance >] Power Options.
   
• On Apple (OS X) systems, standby and power option settings are set under System Preferences.  Go there and then select Energy Saver.

Standby/sleep modes are suitable for when your computer is idle for an hour or more.  A full system shut-down and power-off is appropriate at the end of the work day.  

Will power management hurt my computer?  

It's a myth that turning computers off and on shortens their lives -- unless you turn them off many, many, many times every day.  It's also a myth that starting the computer requires a lot of "extra energy": it actually only takes the equivalent of a few seconds of running time power.

Computers generate a lot of heat -- principally from their central processor units (CPU).  Allowing a "cool down" during a power-off period will generally increase the life span of the entire system.  Allowing your computer to rest its moving parts, like the spinning hard drive, cooling fans, etc., will tend to increase the life-span of those components.

The reboot of the system that takes place when power is restored has another positive effect.  Many software patches and upgrades require a reboot to be fully installed and functional.  A computer that is only rarely rebooted may lag behind on software updates, and accordingly be more vulnerable to malware attacks. 

Is there any downside to power management? 

Obviously you have to consider the value of your time too.  A fully powered-down "off" computer takes a considerably longer time to restore to operational status than one in stand-by mode.  One in stand-by takes longer to restore than one that is fully on -- although not much longer.

We're not recommending you turn your system entirely off unless you plan to be away from it for a long time -- such as at the end of the work day.  We do recommend setting a sleep/stand-by mode for when your system is idle for 30-60 minutes or more.

Unless your system is controlling an ongoing process, such as running/monitoring laboratory equipment, there is usually no good reason to leave it on when you are away for extended periods.  And many good reasons not to.

How does power management work? 

Power management savings come from reducing hardware power to sleep levels when the computer is not fully active.   Idle-ness is defined by an absence of mouse or keyboard activity (and no on-going processes for applications) for a set time period.

Sleep modes for the computer usually involve slowing the clock rate of the CPU and spinning down the hard disk.  While it is possible to reduce energy use in PCs while they are active, much bigger savings come from power management during these idle periods.  Because some of the electronics are kept "warm" in the low power mode, restoration to full functioning from standby/sleep mode is much faster than from a full power-off condition.

Power management in monitors is accomplished by dimming or blanking the monitor -- that is, by reducing or eliminating the beam control power (if a CRT) or the back light (if an LCD).  As with the computer itself, keeping the monitor electronics warm in the low power mode makes recovery faster than from a full power-off.

How much can power you save?  

A desktop PC running at 100 percent can consume in the neighborhood of 50 to 60 watts, or even more.  The exact amount depends on the particular CPU, hard drive and other components.  An idle desktop computer in limited standby mode may consume from 30 to 50 watts, and in a deeper sleep 15 watts or even less.

CRT displays are energy hogs when active -- consuming an additional 65 to 80 watts on top of what the computer consumes.  However, in a low-power standby mode a CRT's power use may be reduced by more than 90 percent.  LCD monitors are much more power efficient -- typically, consuming less than 30 watts when running, and as little as 3-5 watts when in standby.  Note: running a "screensaver" is not the same as being in standby mode.

LCD screens recover from standby faster than CRTs and consume less power when they do.  LCDs do not emit as much waste heat as does a CRT, and this saves energy on air conditioning too.  So, overall, one of the best things you can do for power consumption is get rid of your CRT.

Does a computer or monitor that is "off" consume 0 watts?  Not quite.  A small amount of power is always being used to, among other things, allow a response to you pressing the "on" button.  (It's estimated that as much as 5 percent of US electricity consumption may be from electronic devices that are "off.")  The only way to get to 0 watts is to turn off power at the surge protector.  You are using one of those, right?

What about laptops? 

Laptops are designed to economize on power in order to stretch battery life.  A laptop running at 100 percent may consume more than 30 watts running its processor, display, hard drive and peripherals.  A laptop in standby mode may consume less than 3 watts.

Laptops usually have separate standby configurations for "plugged-in" and "on-battery" mode.  The on-battery settings usually aim for quicker transitions to power-saving modes, for obvious reasons.

More information

Online Guide to Energy-Efficient Equipment (American Council for an Energy-Efficient Economy)
Discussion of potential savings from energy-efficient use of computers and other office equipment

Calculate The Savings Realized By Enabling Monitor Power Management (The Cadmus Group, Inc.) 
Use the simple calculator below to estimate energy and cost savings by enabling monitors for power management

User guide to Power Management in PCs and Monitors (Lawrence Berkeley National Laboratory)
An older treatment of the issues, but very comprehensive 

When to Turn Off Personal Computers (US Department of Energy)
A shorter and simpler treatment of the issues