You can install a script that does that and the report is emailed to you on whatever schedule you wish. Mine comes every morningI strongly encourage people to manually retrieve their HDD temps with Smart using CLI while a solution to make email notification is being rolled out.
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?FreeNAS (by which, you mean, FreeBSD) is not an energy hog. But it does require the user to configure things correctly.