Which main board are you using? 5W is very impressive. I'd be surprised if it was even possible to achieve with an ATX board without some serious tuning.
I'm curious to know the details of this as well. 5 watts is absurdly low for an ATX system. I've definitely seen 25 watts before. Assuming you went from a 75% efficient PSU to a 90% efficient one, that'd shave off a few more watts but it won't get you that low.
For perspective, I think the typical NIC consumes several watts of power.
I didn't do any significant customizations, it runs Alpine Linux. Here is my powertop output:
The system as a whole spends around 95-97% of time in pc8 state.