Telecom towers and their power

I assume cell phone towers have generators, and battery backup for when the generators run out of gas.

Does anybody know if there is a standard on how long they have to be able to provide power for?

While power was out for my relatives in Florida this morning and Friends in Charleston, SC early afternoon, I got a few texts saying they were safe and wet(SC), now texts are returned unsent, and calls go right to voicemail.

I imagine no internet ,no power, No airconditioning, no smartphone data, has a whole bunch of people In SE US contemplating a lot of things always taken for granted, well Those who still have livable structures to reside within.