For something approaching 20 years, I’ve used OpenBSD to firewall my network from the internet and provide basic network services (DHCP, DNS, NTP, VPN, etc.). Just recently I’ve decided to retire OpenBSD and stand alone computers from the role of firewalls for something smaller, lower power, and easier to manage and upgrade.
I’ve been steadily moving towards smaller and lower power systems for as long as I’ve been doing OpenBSD based firewalls. My first machines were nothing more than mid-tower desktops that I had upgraded away from. In 2000-2003 I made my first moves towards building something more specialized, when I switched from using old towers to building a specific micro-atx pizza box style machines; though still with standard Athlon XP CPUs and parts.
In 2010 I replaced the micro-ATX Athlon XP with a mini-ITX based Intel Atom D510 machine. This halved the power consumption, from somewhere around 80-100 W to something closer to 40 W.
Around 2015 or so I started looking into running OpenBSD off a USB flash drive instead of a standard hard drive. Part of this was to remove the power consumption of the HDD from the equation. In this, final configuration, the D510 machine with 2 NICs and 2GB of RAM turned in at a somewhat respectable 30 W. Though that was hampered by an abysmally bad PSU with almost 0 power factor correction that pulled nearly 60 VA.