Theoretically, what would be the most power efficient way to run the HL15's backplane?

HBA’s use a lot of power & have inconsistent sleep state support depending on your system. I’m wondering if it makes sense to use a motherboard with 4 or 8 sata cables & use those for 25-50% of the backplane & then use a cheap sata pcie expander card to add the other 50-75% of the connection. You could even use an m.2 to sata converter. You might get some latency penalties by going through the chipset, but I don’t think that would be too much of a problem when only using hard drives. What’s the most power efficient HBA that would work with the backplane?

I believe the Full Build is wiring the backplane into the X11-generation board from Supermicro rather than eating a PCIe slot for a separate HBA, however, I can’t see how the X11 board is going to be dramatically more efficient to run a controller on the mainboard vs. a HBA in the PCIe slot?

I should have specified that I meant standard issue, off the shelf motherboards without the supermicro special features.

I’m also very interested in this. Any ideas yet?

Assuming the motherboard has some SATA ports already, probably something like this;

[https://www.amazon.com/12-Port-PCIe-SATA-Expansion-Cables/dp/B0C552HPBR/ref=sr_1_10

would have the lowest total power draw for the SATA interfacing.

You have to be aware that you are making a huge trade off in speed and reliability by doing this, though. These cards are typically PCIe v1 x4 or PCIe v3 x1, so are limited to the speed of 4 SATA HDDs by the PCIe. You won’t be doing concurrent operations across all the drives without a lot of bottlenecking (ie, expect an 8-disk or 15-disk vdev to be very under-performant), and the chipsets used aren’t particularly stable under heavy loads, so expect drives to drop out occasionally.