This has been discussed at length in the past but briefly, you need to decide what you're trying to protect your tank from. Then, you need to know what minimum systems you can get away with. Then you need to know how much power those systems take. Then you need to decide how long you might want to run those systems. Then you need to size a UPS to match.
Here in Calgary, it is VERY rare for a power outage to last more than about 30 minutes. Most tanks can survive with nothing running for that length of time.
In the event of a serious outage (say, a day or more), then you're not interested in having something kick-in immediately and run for an hour or 2 and then die; you want something you can run for maybe 30 minutes every 2 hours (i.e. not a UPS)
Of course, if you're around, then a generator is an obvious solution, but what if you're not at home ?
If you have a really big investment in a large system then what you really want is:
a) a UPS
b)a monitoring system (connected to the UPS) to alert you to a power failure
c) a generator.
Then you've got the bases covered. Probably.
