God I miss my old opteron box.

If you're running anything more than about 5 years old you're just going to chew through power for the sake of chewing through power. You can make little reductions here and there but the entire architecture was all wrong for power savings.

A 10th gen intel i3 on a desktop motherboard will absolutely beat the pants off of older server boxes, that and getting away from spending media or the two biggest savings.

Opteron, what year is this?

Those old server CPUs and motherboards just draw a ton of power, I'm not sure there's much more you can do.

It might be best to change hardware if you need lower power draw, a $50 PC off ebay with an i5-7500 should be faster and a only uses about 15W idle.

I dunno about Linux, but on Windows I used to use something called K10stat to manually undervolt cores with no access to such via the BIOS. The difference was night and day dramatic, as they idled ridiculously fast and AMD left a ton of voltage headroom back then.

I bet there’s some Linux software to do it. Look up if anyone used voltage control software for desktop Phenom IIs and such.

If you've optimized your BIOS settings (balanced mode or power saving wherever possible), the only other option is removing extraneous hardware. All hardware power use (disks, HBAs, other adapters and controllers) adds up. I managed to get idle power consumption of an HP DL-380 G9 down to about 60w (started at 210w) by removing the disks, RAID controller and battery, fiber channel adapters, and extra Ethernet adapter. Each SAS disk I removed saved me 10w. I used one M.2 drive in a PCI adapter instead.

Like you mentioned, these aren't designed to save power. That Opteron (and the chip set) hales from a time before "performance per watt" was a thing.

Tangential question: What kind of server apps require that kind of processing power? I run a server on an Intel N200 laptop with multiple apps and services and it rarely uses more than 12% CPU and 15 watts. I'm wondering if I'm going to eventually run into something that needs a more powerful platform.

I doubt this would fit your use case but wake-on-lan could keep power draw stupid low when nothing's being used, at the cost of boot time.

What other hardware does it have? HDDs draw a lot, GPU of course, but also each active network interface etc.