Benefits of a RAID BBU in addition to a double UPS + PS system

It doesn't power the disks, it just keeps the data in the cache for (in this case) up to 72 hours until you bring the machine back on line. When you power the machine back up it will write the contents of the cache back out to the disks.

All it does is protect against a power failure. If (for some reason) the machine loses power without cleanly flushing the data out to disk the battery keeps the cache contents alive until you can restart the machine.

It is not a UPS for disks, as the disks could be in an external disk array, or even on a different power circuit. Even a UPS could fail.

1. Sizing - For a rough estimate you can look-up the maximum power ratings of each device, assume they're running at 75% normally and calculate the power draw.

   If you want something more accurate, use a Kill-A-Watt type device. Some servers and computer have management information that can tell you how much power they're drawing. Many "accessory" devices such as monitors draw full power constantly.

   Offhanded guess, you're probably around 1-2kW.

2. Assuming the 2kW guess is reasonable, you could get a single UPS that will run these devices for 10-15 minutes (multiple UPSs may be logistically more feasible). Which is enough time to shutdown. You can buy a portable generator that could run them for an extended amount of time. You'd probably want a permanent automatic generator if you want uninterrupted service, as it generally takes more than 10 minutes to get a portable generator running and all.

3. Adding more UPSs for more devices is easy for the scale of your situation. Adding more generators is not so easy, you'd probably want to upgrade the generator as demand increases. See a proper electrician for "future-proofing" your plans, expect to pay a lot more for capacity that you don't need "right now".

4. Batteries store much less power than most people think. You'd need a batter bank the size of a cubicle to run all that for "several hours". (Caution: some hyperbole present in the size estimation; but a system large enough for "several hours" is going to be rather large).

5. What you "need" is up to you... USP is the only stopgap between power-out and generator. Generators are the only realistic way of keeping the power going for more than "minutes". An automatic generator and ATS (automatic transfer switch) is the only way to provide reasonable assurance that service isn't interrupted when the power goes out.