I'd check the BIOS to see if there are any power saving options, and/or unclocking/undervolting the CPU. 2.5" HDDs and SSDs use about the same amount of power,

I'd check the BIOS to see if there are any power saving options, and/or unclocking/undervolting the CPU. 2.5" HDDs and SSDs use about the same amount of power, but 3.5" HDDs use 2-3x more. So you could potentially save 12 watts if you're using 3.5" drives. Make sure your PSU is a high efficiency one, and isn't too low, or or too high wattage. From what I've read the maximum efficiency is in the 40-80% range.