I am using a couple of products together for this. My system is mainly ip cameras which use POE, and PIR sensors running security system on the pi.
I used one of these for the battery backed power “CCTV 12V 8A 9ch Switching Power Supply with Backup”
Together with “LCD Display DC Buck Step-down Voltage Converter Regulator 5v-23v” configured for 5.2 volt (allowing for voltage drop with thin power cables - I cut down a thin phone power cable):
This runs the pi3. Then, outside of the scope of the question but I added a “DC 12V to 48V 4A 192W Step-up” module to get 48v for POE
And connected that in to a POE smart switch. This battery backs the cameras too. Several VLANs and a pfsense router allow me to isolate IoT devices and cameras, and the TV, (all I consider untrusted) and only allow the TV out to the internet for streaming content and firmware updates. My trusted network PCs can connect in to the other vlans to use those devices, but not the other way around.
Of course you could just run a normal mains voltage UPS to the pi but I had a lot of fun building this and I think it doesnt hurt to consider security at the same time if the HA components tie in to your security system.