I have an old pentium laptop N3520 with 4GB DDR3. I removed everything and put it in a receiver box 1U size. It consumes so little energy that it can run 5-7 hours from its battery (I call it build in USV). Last time I measured 3-7w. Also passiv cooled , no noise. Another machine I use , is with a i7 4770 with 16GB for Proxmox, 7-20w , peak is much higher but rarely used , only on boot and vm startup.