would be a good idea to test the backups on 7, and double check the release notes, they hold just about every caveats. The 7to8 upgrade was not horrible, if you have backups, you could always attempt the upgrade after taking backups, then if successful take new backups, test, then install new drives and restore. depends on how paranoid you feel.
If this was a production system, that is probably the change plan i would follow, but in production I would also be able to migrate VMs. I am not nearly as careful in the home environment.
make sure it’s configured for clean shut downs before your battery runs out, auto power up on restoration, and hope it doesn’t happen. you will eventually have an outage that outlasts your batteries.
I have a large string of batteries from an old telco office, that runs my rack for 14hrs (calculated, I shut everything down around this time) and that did not last for the 2-3 day outage we had after a storm. Without a generator, you will inevitably have an outage, but if you are prepared, then you can mitigate any damage. use NUT if you need to shutdown or power multiple devices from one monitored UPS