Save energy by reducing the processor speed of your computers when not in use.
I've tried lots of different power management techniques with linux. I was successful in getting hibernate to work with suspend2 sources on my Toshiba laptop (howto to come) and finally I got Power Now to work!
For my system, I loaded the following kernel modules:
Though I guess I don't need it, I could not modprobe acpi-cpufreq:
FATAL: Error inserting acpi_cpufreq (/lib/modules/22.214.171.124/kernel/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.ko): Device or resource busy
Unfortunately, the lower processor setting only saves a few watts. Better than nothing. Especially for a machine that is on 24/7/365!
I am also trying to get my UPS into the loop. I have an APC Back UPS 500, connected via USB 2 Ethernet, so I'm going to try apcupsd. I did get apcupsd to install and run. I'm very impressed so far!
This machine uses a software raid, and the performance is amazing. However, since this machine is on 24/7/365, I'd like to save a little juice at night. I was weary about using hdparm for sata drives configured in a raid, but it seems kosher. I DO NOT ADVISE YOU TO TRY IT! These are just my thoughts.
Laptop Mode Tools FAQ is helpful: http://www.xs4all.nl/~bsamwel/laptop_mode/tools/faq.html
Using the trusty Kill-A-Watt meter, I was able to check the difference between standby disks and active disks, and it is over 20 watts. I'm definitely going to set that up. Right now I can't because we run Asterisk and Nagios on the same server, which writes to the drive fairly often. I did set the commit time for the ext3 filesystem to 7200 (two hours), and am also using laptop-mode-tools. I think this is doable.
Many sites have explained that constantly spinning down and spinning up your drives causes them to fail faster. I agree with this statement, but seeing that heat can also hurt computer components, spinning down the drives once a day should be fine. I would shut down the computer alltogether, but that would then prevent me from being able to access the server if I need to.
These websites were very helpful for ACPI:
And this one for apcupsd: http://www.apcupsd.org/manual/