>I love FreeBSD and was planning on switching some servers from Linux but found that it consumed about 25% more power than my Linux boxes

Wow, I wouldn't have thought there would be such a dramatic difference in power consumption on anything but portable hardware. Did you try to measure the CPU and I/O usage on your servers and compare them between Linux and FreeBSD? I wonder if FreeBSD's greater power draw was due to its ACPI implementation or if the average system load was also higher (ZFS?).

I wasn't using zfs at the time and I didn't do much comparison with CPU and IO, I used a wattage meter. I followed the FreeBSD guide (http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/ac...) and tried just about everything. My Linux box was running at about 9watts (idle) and freeBSD started out at around 18, but I got it down to about 12watts after making a lot of optimizations.

I think most of the difference is because FreeBSD is using older more stable software, and the Ubuntu server was running some of the latest software.

Do you remember which FreeBSD version and which Linux kernel version was it?

Linux had tickless kernel + CPU scaling for a while. FreeBSD is lagging in this aspect.

The FreeBSD kernel is tickless as of FreeBSD 10:

http://www.freebsdnews.net/2013/09/20/freebsd-10s-new-techno...

Yes, exactly, it only has it for couple of months, that's why I asked about version.

I'd be interested to see some benchmarks comparing Linux and FreeBSD power usage. Personally I've been impressed w/ power management on FreeBSD 10 using recent hardware - once I enabled powerd and set the powerd_flags - but I haven't installed Linux on the same hardware.