Total watts per hour divided by 12 multiplied by hours required equals Amp  storage required.  

For long deep-cycle  battery life  50% to 70% of capacity is the usable storage depending on choice of battery quality.

Criteria for assessing the need for an Uninterrupted Power Supply (UPS) function

​The need for a UPS or backup supply will depend on whether the client decides if the use of his equipment/appliances is essential or not. It is here that some clients may need to be advised of the costs versus the "needs", as these could become too prohibitive.
Some questions and pointers to assist  in determining what product is best suited to meet individual needs:
•  What is the budget?
•  What are their critical or essential needs, and does their budget allow these needs to be met? A trade- off  between perceived need and critical need must be made with reference to budget constraints
•  How sensitive is the equipment or the appliances to "noisy" or unstable supplies, e.g, spikes, surges, dips, irregular supply, etc.?
•  What unit is best suited to meet their critical needs (i.e., is only enough time required to shut down computers or other appliances to continue operating for the estimated duration of the power outage?).This is another important consideration, as the longer the backup time, the more expensive it becomes
•  Where to position the UPS and associated equipment.
•  Do they want it to operate through the electrical installation or is it going to be plugged into a socket outlet, i.e., do they want a DIY type unit or something more sophisticated?