A google search of "UPS battery discharge calculation" produced a number of links to suitable calculation webpages and the formulas behind it (depending on how accurate you want the answer).
You have given 10 batteries and each battery is 20V/12amp hr. Total voltage is 230V.
And I presume that you connect your batteries in series connection. But in series connection the total voltage is 10 x 20V = 200V.
If you need to calculate how much time it takes to discharge completely at o/p load of 10% we need to know your total load. So what is your total load?
Asking a question like this, it is impossible for anybody tell you, how to calculate the time taken to discharge completely at o/p load of 10%. 10% of what?.
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The original poster did not state the load on the batteries, therefore this calculation is not valid!
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well what I thought that the battery capacity includes the battery load .offcourse I agree that some energy is lost across the battery which is constant but the battery specification do includes this losses .