[zekica]

You should really look into Intel Pentium N6005 based mini PCs with up to 5 ethernet inferaces - they are very versatile and you can choose from different Router systems: OpenWRT, pfSense, opnSense and others. They consume less than 10 watts idle (most of the time) and about 25W max. Regular consumer routers consume 3-5W idle and about 15W max, so not much of a difference. You can even use a hypervisor and use VT-d to pass through the ethernet cards to the router VM. The only downside is that there are a small number of PCIe cards that work good as an wireless AP - mostly Mediatek based ones (some Qualcomm/Atheros cards work fine too).

[dtaht]

I kind of gave up on using the central box as a wifi router, and just put bridged APs closer to wherever they are needed. Simpler, and you can do more stuff on your firewall that way (like use it as a NAS)