The best things are always the most simple things.

I added a UPS solution (as we have quite ofther power outages) to the smart home.

Well, connect a simple USB power bank to the unit (between the USB charger and the arduino unit), and you are done. Simple? Yes it is. Works? Yes, it does. 

I added a small code to detect if the system is running from battery - in that case the backlight of LCDs are cut off as they drain quite much power.