Diesel generators are great if you need a few hours of backup (assuming the generator actually starts when you call on it).
But if you need enough backup capacity to survive something a multi-state, multi-day blackout [1] that probably gets expensive.
You wouldn't need that for a premium erotic call processor, but a 911 call exchange might, for the portion of their workload they can't pass off to another exchange.
In many places going over what you can store might put you over emission quotas and you would have to shut down anyway. I'm familiar with one incident at large DC which had fuel left and could easily get more, but only had few hours before they were required to shut down by EPA.
I worked at a site where we powered a bunch of stuff consistently from diesel generators until the grid hookup was finished, much longer than a few days. Probably was expensive.
But if you need enough backup capacity to survive something a multi-state, multi-day blackout [1] that probably gets expensive.
You wouldn't need that for a premium erotic call processor, but a 911 call exchange might, for the portion of their workload they can't pass off to another exchange.
[1] https://en.wikipedia.org/wiki/Northeast_blackout_of_2003