Second wifi router power consume < 4W

It not the main router as home gateway, only serve one room, with 802.11ac and giga ether x4 port swither mode VLAN, and support LEDE/OpenWRT.
Because I am using 100Wh router DC12V UPS for router and webcam, I want a low power consume router < 4W for longer online time when home AC electricity fails.
Any brand and model suggestion?

The GbE requirement may make it difficult. If it weren't for that, I'd suggest something like an Archer C59. You definitely want Atheros chipset.

If MT7621 only using 5G MT7612E, avg 5W~6W, I am still looking for better way.

with all ethernet ports active you will not find something that has so low power consumption

this one could be very low power, but not performance, only 100mb/s

When home AC power lost, surly not all Gb LAN port will be used, at lease PC and NAS and TV all down. I hope the router work in router UPS DC12V mode < 4W, mainly provide 5G Wifi and webcam LAN port in room.

When PowerTOP was a hot topic in the Linux world, a few years ago, there were talks that configuring GbE cards to only negotiate up to 100Mbps could save "significant" amount of power. See for example
No idea if this is still true today, and whether these configuration tools are available on LEDE.

Many router do not using single port NIC card except x86 soft router, they are using switch chipset which even have VLAN and HW NAT features, typically combine with 4x LAN ports.
I think it possible to set that switch chipset work at 100Mb low speed, but it may unnecessary because my GbE devices all down while home AC electricity fails, the webCam only have 100Mb LAN links, so my router working at 100Mb mode already, of course the upper link to the primary router still 1000Mb. But my router DC UPS can not give my router a AC fails signal, it difficult to detect the opportunity to switch that upper link port from 1000M to 100M.

Phicomm K2P HW:A1 with one giga LAN uplink and one 100M webcam LAN link, 5G on / 2.4G off, LEDE r4426-80d9ec5 kernel 4.4.71 and MTK reserved source MT6715 driver. router power consume < 4W average.

Turn on Energy-Efficient Ethernet IEEE 802.3az on this router (ap mode) and uplink router, will save more power on idle time.