Why bother with a UPS for the ONT? What other equipment do you have that uses the internet connection and will work in a power cut?
For me, it's three-fold:
1) My UPS (230v output) powers router, VDSL modem (sadly no fibre here yet), Home Assistant, couple of switches and the CCTV PoE and NVR. This means that I get uninterrupted security monitoring of the house during a power cut, at least for as long as the batteries hold out
2) Most power cuts (round here, anyway) are less than 10 minutes long, so I don't get any service interruption for a normal power cut. Nothing reboots, so nothing has to start up cleanly. Much neater than having a home router crash back into life after a 2 second glitch. Also, I get notified that the power has gone out which is useful in knowing that I need to head home and take precautions against things like a defrosting freezer.
3) The exchange has backup diesel so should run uninterrupted for far longer than my own UPS will (around 45 mins).
Since I put this UPS in, I've had a few power cuts but none have gone deeper than 50% of the UPS battery before restoring power. Definitely a very worthwhile investment and *definitely* worth using with the ONT / VDSL modem to protect the internet connection, at least for me.
Edited by daern (Thu 21-Mar-24 17:01:01)