It might be a guaranteed for couple of seconds/minutes, but not necessarily for 1-1.5 hour power outage, depending on type of the battery and power load of the communication hardware.
This thread is getting very OT now, but your statement is incorrect and I'm not sure where you're got that information from.
Even in Christchurch during the earthquake there were very few cabinets that went down, and I don't believe there was a single exchange that actually lost power.
As for "defending yourself" I'm not sure what you're trying to defend yourself from. You requested a tech visit to diagnose a fault. No fault was found, so you were billed accordingly. It's a very clear case.
Yes you may disagree with the fault diagnosis and the pricing, but that's the reality of the world we live in. The facts around broadband faults are very clear - the vast majority of problems are caused by issues within the customers premises - on cabling that is owned by the end customer, not Chorus.
If your situation I've seen nothing so far to suggest a Chorus issue. There is plenty of debate in the RSP world over Chorus closing jobs with a NFF and faults mysteriously being fixed at the same time, which some simply see as Chorus ripping off RSP's, but based upon what you've said there is nothing to suggest that is the case.
A tech does not need to a visit a site to determine a fault - testing at the demark or pillar probably showed the exact same sync speed.