Oh, providing backup power is a VERY complex topic. It's a LOT more complex than simply buying/renting a generator.
Consider that, during a wide-spread power outage, generators disappear at Warp 10 speed. Thus, if you're going to be using one, it had better either be bolted in place at your building well before the event, or it needs to be provided for in a long-planned rental contract with a company who is reliable (probably with clauses which make it too painful for them to renege). And, for anything under about 10 tons, it needs to be locked in place, else it may wander off in the middle of the night.
Of course, simply having a generator won't do much good unless the building wiring is set up with an appropriate transfer switch, which will need to be installed by a licensed electrician in most jurisdictions.
One probably doesn't want the entire building to be powered from the generator, well, unless one wants a really huge generator. So, usually only essential services are powered by the generator. However, the definition of essential varies considerably. Does the room lighting need to be powered, or will the employees be in the dark (literally)? What about the HVAC system? Will the employees be sitting in a tropical jungle environment, or will they be shivering in arctic-like conditions? Will the vending machines be powered, or will the staff be forced to do without coffee? How about workstations? What about networking? What about the phone system, both inside and outside the building? Of course, all of these decisions will impact the electrical system in the building, and may require substantial rewiring.
I've seen some installations which have only powered the machine room. Of course, the first black-out that hits leaves the programmers and support people in the dark, with their workstations shutdown, and no way to support/administer the servers. Whoopsie. I've seen other installations where some bleary-eyed fool plugging in a coffee pot brings the entire generator to its knees and shuts the entire building down. Whoopsie.
There are also safety concerns with where the generator is placed. If it's on the top of the building, can the building withstand the vibratory load it will produce, or will the building come down like a pile of sticks? I'm aware of one roof mounted generator which had fuel supply problems, since the fuel tank was in the basement, and the lack of commercial power meant that the fuel pump couldn't be run to get the fuel to the generator. Whoopsie. Do you really want your employees carting 55 gallon drums of fuel up the stairs?
Also, consider where the generators are vented. Are they vented into the air conditioning intakes for the building? How long before the staff keels over from Carbon Monoxide poisoning? Note that some of those generators have six inch (or larger) stacks on them. That's a LOT of exhaust.
Back to the subject of fuel, will there be fuel available, and of the correct type, for the generator in the event of a wide-spread power outage? Note that most gas stations won't be able to pump fuel in the event that there's a wide-spread power outage (Their pumps run on electricity, too.). But, if storing fuel on-site, can it be done safely? 1000 gallons of gasoline (or even diesel) will make a VERY impressive column of flame if it gets ignited accidentally. Plus, there are legal restrictions on where fuel can be stored in some jurisdictions. Some make it illegal to store fuel in an occupied building. Others may have rules about tank leakage safety. There's the whole insurance thing, too.
Also, can the fuel be stored for long periods of time with out jellifying (polymerizing)? I've seen 5 gallon cans of gasoline that has been stored for a year look more like jelly than a liquid (Hint: Generators don't like trying to burn jelly!). There are products which supposedly keep gasoline/diesel from jellifying, but they have to be bought and added to the fuel.
Oh, yeah, that reminds me. Don't forget to periodically test the installation. It's no good having a backup generator if the thing won't start. Or, if the transfer switch is fused/stuck. Of course, such testing should be done in a manner which won't take the building down if something doesn't work. There's also regular maintenance issues with the generator, such as changing the engine oil. Or, for that matter, even checking that the think has oil in it. There aren't bird nests in the exhaust stack, are there?
I could go on and on (and probably write a book about the things I've seen done correctly and done wrongly), but I think it's clear that providing backup power isn't a simple, easy, nor cheap task. It can be done correctly, but only with the correct people planning and maintaining the installation, and enough resources to do the job correctly.
P.S. I'll get my coat; it's the one with the package of AA batteries in the pocket.