This is not enough data to give you an answer.
To get the capacity of your battery you also need the voltage its operating on and the batterys discharge rate. The discharge rate, unless you directly get a X watt, is oftengiven in C, which represents the number of full charges taht could be giving of in 1 hour (i.e. 2C -> 0.5 hours to discharge).
As an example for a 12V battery at 2C discharge rate you get:
100 Ah/0.5h = 200 A
200A*12V = 1200W
On a sidenote: The takeover from main power to battery needs to be FAST, on the order of miliseconds