I have a 5000 VA UPS which is connected to eight 65 AH batteries. I am looking to replace these old batteries with new ones. I was wondering whether the UPS will run safely with less number of batteries or with less battery capacity.

I did some research and found out that the formula for calculating the battery capacity goes something like this:

Battery capacity = (Load / Voltage) * Back up time

I am assuming that the load is 5000 VA which will roughly convert to some 4000 Watts and its a 12 V battery. The back up time required is 30 minutes.

So battery capacity = (4000 / 12) * 0.5 = 166.

Is my formula wrong? I don't have an electrical background. Any help would be appreciated. Thanks.


1 Answer 1


Your formula establishes the minimum A-hr battery capacity. 166/65 = 2.55, so an absolute minimum requirement is 3 batteries of 65 A-hr each. However, if you want long life (in terms of charge/discharge cycles), standard lead-acid batteries should not be discharged more than 50%. This suggests that you should use a minimum of 6 batteries.

If you run the same calculation for 5000 watts and 30 minutes, you'll get the 8 battery requirement which seems to have been realized.

  • \$\begingroup\$ Got it. But if instead of using 65Ah batteries, if I were to use 26 AH batteries, the min. requirement would be 166/26 = 6.3. So if I use fourteen 26 Ah, will it achieve the same result? \$\endgroup\$
    – Depz
    Commented Nov 9, 2017 at 2:13
  • \$\begingroup\$ @Depz - You got it. \$\endgroup\$ Commented Nov 9, 2017 at 2:43

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