Very similar to my recent build - E3-1245v5 / Asus P10S-WS (C236), 1x16GB ECC, 5x4TB WD Reds, 240GB SSD, Seasonic X-660 PSU in a Fractal Define XL case. Mine idles at 33-34w at the wall with all drives spun down, quite a bit more than your 18w, and I'm not using a RAID card. Wonder if it's the PSU or M/B contributing the difference?