The backup battery for the FIOS system. Should give at least 8 hours of service with the electricity out. It's a sealed wet cell battery. Replacement batteries run roughly $20-$50, and as I noted, they need to be replaced every 5 years or so. (They start beeping, you can re-set them a few time to cut off the beeper). https://www.verizon.com/support/residential/battery-backup
OK, yes, I am aware of that. As I stated, we still haven’t installed it because it’s quite inconvenient to our house layout and furniture situation.
You’re right, it probably works for most failures. But, several times even with our underground system, major ice/blizzards and hurricane weather has knocked out power for days. But not enough for me to really worry about it.