Any tiny/mini/micro systen, get one second hand from EBay, get the Allstate insurance option in case it turns out to be a dudd, and slap opnsense on that thing. Most of their energy pull will be sub 15w, many even sub 10w.
HUGE EDIT: Ensure that you get a mini PC with at least two Ethernet ports or one that you can put a nic into, but finding form factors with a pcie slot will be harder. If you need a nic for specific reasons (10Gbit, or fiber, or something else) stick with a mini tower form factor instead of a tiny/mini/micro form factor. Those almost all have multiple pcie slots for expansion.
Any tiny/mini/micro systen, get one second hand from EBay, get the Allstate insurance option in case it turns out to be a dudd, and slap opnsense on that thing. Most of their energy pull will be sub 15w, many even sub 10w.
HUGE EDIT: Ensure that you get a mini PC with at least two Ethernet ports or one that you can put a nic into, but finding form factors with a pcie slot will be harder. If you need a nic for specific reasons (10Gbit, or fiber, or something else) stick with a mini tower form factor instead of a tiny/mini/micro form factor. Those almost all have multiple pcie slots for expansion.