UPS provides power backup for a short period of few minutes only.We want DC power backup for max/entire duration of power failure.Any More Inputs?
A battery powered UPS can last more than "a few minutes" if the battery size is sufficient to provide power for a longer time.
What is the power draw on the unit to remain with power available? How long is the power outage expected to be? You cannot plan on a battery backup system to last "forever" and cover ALL power outage scenarios. Is the battery supposed to last an hour, a day or a week?
A 325VA system may indeed last a few minutes with whatever you have plugged into it - and if you can afford a 5000VA UPS, it will last 15x longer… And if you have the space and knowledge and requirement and skill, a 250 kV system will last WAY longer…
http://apc.com/products/category.cfm?id=13&segmentID=5
Most large scale systems would have a generator kick in - and you need to keep if fueled and in working order. You will want a serious power conditioner to protect the items using power from getting burned out by the surge when the generator kicks in.
Another option would be to have the battery array be sized to work for 12 hours - and a solar panel array that is designed to recharge the batteries during the day.
Large scale DC power systems, sizing, design and maintenance are not "do it yourself" homeowner activities. The carry a lot f power (enough to kill a person) and some serious training and experience is needed before taking this on. If you are that serious about this, there are many power professionals you can contact to help you through the maze of requirements.
APC is a good place to start… http://apc.com