Nominal Cost of Standby
On the job one day we wondered about how much
power a computer system uses while in “sleep” mode.
question was intriguing from a work-related view and from the viewpoint
of paying utilities customers in our own homes. We decided to
On the test bench we had an isolated power
supply, a 17” CRT monitor, a 15” LCD display, a
PC (tower), and an Apple iMac. The plan was to test each
separately to accurately display the results and, hopefully, to
demonstrate various cost-saving measures. The specific
each device will be listed following the test results for those wishing
to make their own comparisons.
The displays (and iMac) were tested as
• No signal
• Default Desktop
• Black Desktop
• White Desktop
The PC tower:
• Under Benchmark utility
The iMac underwent the same tests as the
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.
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
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.
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
display activity when the system is unattended for a given period of
We chose these systems out of what was
available to us for testing and find them to be a reasonable
approximation of the “average” home
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
of Test Equipment and
400 MHz PowerPC G3
256 MB RAM
256 MB RAM
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
delivery totaling $0.057568 per kW hour. Using the test
as an approximation of Kevin's home system he determined it would cost
roughly $0.14 a day to let his system idle.
Updated: 8 April 2005