When the two 4TB HDDs I added are in spindown the system uses around 40W when idle vs ~55W when idle running FreeNAS with activated C-States and smartd. That's quite a huge difference. What else could one do to bring down the power usage of FreeNAS?
I suggested the usage of C-states to reduce consumption a bit back in 2015 (according to an Ubiquiti mPower average consumption for a Microserver Gen8 went roughly from 47 W to 45 W, but Cyberjock warned that using C-states can sometimes lead to problems.
Not all hardware supports them properly. (Peecees, shocking!)
Just A FU for this topic since I couldn't get significant results on the previous platform (2x L5420). With a more modern Xeon E3-1220 , I see a decrease of about 18-22w out of a 100w base load, so I'd say C-States in FreeNAS definitely work!!