The Nominal Cost of Standby

Nick Bradbury
Kevin Lisowski


    On the job one day we wondered about how much power a computer system uses while in “sleep” mode. The question was intriguing from a work-related view and from the viewpoint of paying utilities customers in our own homes.  We decided to find out.


    On the test bench we had an isolated power supply, a 17” CRT monitor, a 15” LCD display, a Dell brand PC (tower), and an Apple iMac.  The plan was to test each device separately to accurately display the results and, hopefully, to demonstrate various cost-saving measures.  The specific details of each device will be listed following the test results for those wishing to make their own comparisons.


    The displays (and iMac) were tested as follows:
•    Off
•    No signal
•    Default Desktop
•    Black Desktop
•    White Desktop

    The PC tower:
•    Off
•    Sleep/Suspend
•    Idle/On
•    Under Benchmark utility

    The iMac underwent the same tests as the independent displays with the addition of a benchmarking utility to directly compare with the PC.
All tests were conducted with a regulated supply of 115 volts.


Chart showing watt usage of 17inch CRT monitor.

Chart showing watt usage of a 15inch LCD

Chart showing watt usage of Dell tower PC

Chart showing watt usage of iMac system


    The 17” CRT had minimal draw in both the “Off” and “No Signal” tests and roughly two times the consumption rate of the 15” flat panel in the other tests.  We noted that the choice of background color made a significant impact on power consumption in the CRT models due to the increased activity in the electron guns drawing the display.

    A severe disparity was present when comparing under benchmarking loads between the iMac and the PC (when factoring in the addition of a CRT monitor).

    Neither system experienced significant draw while in a Sleep/Suspend mode, however there was a notable lack of effect by placing the systems in a “ready” state, i.e. standby or idle mode.  The amount of change between an active and standby/idle mode were too minimal to even warrant a plot on the charts.

    Further gains could be made by managing disk and display activity when the system is unattended for a given period of time.

    We chose these systems out of what was readily available to us for testing and find them to be a reasonable approximation of the “average” home user’s equipment.

Additional Questions

    Some questions yet unanswered:
•    How much does it cost per day to leave our systems on?
•    How long would it take for a LCD display to “pay for itself” from power economy?
•    What impact do these results have on our workplace?

Specifications of Test Equipment and Systems

    Dell Optiplex GX260     Apple iMac
    1.7 GHz Celeron            400 MHz PowerPC G3
    256 MB RAM                256 MB RAM
    Onboard Video

    Dell Branded Monitor            Sencore PR750
    Basic 17” model                       Variable Isolation Transformer


Checking into the practical application of this testing we got the rates for (residential) power and delivery.  Those rates are $0.030349 for power and $0.027219 for delivery totaling $0.057568 per kW hour.  Using the test results as an approximation of Kevin's home system he determined it would cost roughly $0.14 a day to let his system idle.

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Last Updated: 8 April 2005