Cpu and ethernet ports consume more !
With very similar hardware (same SoC) except that it has 5 ethernet ports and a USB port, the hAP ac^2 is stated as "Max consumption without attachments 16W"
I'd guess each gigabit ethernet port drains roughly a max of 1W, not taking into account the USB power requirement...
As someone else answered already, this is a maximum for a lot of CPU and comms activity. My hAP ac^2 in small setup (~10 devices, not too much traffic) works very cold with modern firmwares that control the CPU speed. Using 7.1beta3 and "auto" cpu frequency it is running most of the time at 448MHz with the occasional jump to 672MHz. CPU use is 0% most of the time, jumping to 1% when the clock speeds. The range of values is
[admin@MT] > /system/routerboard/settings/set cpu-frequency=<TAB>
448MHz 488MHz 512MHz 537MHz 565MHz 597MHz 632MHz 672MHz 716MHz 768MHz 827MHz 896MHz auto
Of course if I start uploading encrypted data through the VPN things change.