I don’t know what electricity costs in your region, but I consider that as part of my decisions. You can soon spend more on power than the device costs.
A router will be on 24/7 with a fairly consistent load (sure there’s peaks, more in the day, etc…)
Personally, I’d recommend you try a few different options - an old x86 PC is terrible for power, but great to try out pfSense / OPNSense / DD-Wrt / OpenWrt / Tomato first… then from that pick your real hardware.
Personally, I’ve tried a few things over the years and the *WRTs in a single re-imaged old wifi-router is probably everyone’s Step 1.