Calculate right Battery size for your UPS


Today in this post, we will tell you about choosing the right battery size for your UPS. It should be remembered that finding the right and appropriate battery for your UPS is not such a simple task as calculating UPS power rating.

In order to understand it easily, we will show the guideline to calculate battery size for your UPS.


  • Load is the power used by your UPS.
  • Runtime is the backup time of your batteries.
  • Input is the battery input voltage of UPS.
  • Ah is the number of amperes provided by battery in one hour.
  • Power factor and UPS efficiency are actual power loss and the usage of actual power respectively.
  • Charger Speed should also be given importance as large batteries can’t be charged by UPS in short amount of time.

Ah rating and battery capacity are directly proportional but the Ah rating heavily depends on factors like temperature, type of battery and load on the battery. The manufacturer calculates Ah rating under a certain runtime. If you have a battery of 120Ah then it is tested for 20 hours with load current of 6A till its end. But if you double its load to 12A then it will work less than 10 hours.

Calculate Battery Size

We can know about our required battery size by using few formulae. Here below, we take an example:

  • Load = 1000W (Can be calculated using our previous guide here or it can be max UPS capacity)
  • UPS Input = 24V
  • Required Runtime = 1.5 hours
  • UPS Efficiency/Power Factor = 70% or 0.7 (this should be used if you don’t want to estimate it)
  • Battery type = Lead-Acid (So runtime factor of 1.5)

Load Current for 1 hour = (Runtime Factor x Load) / (UPS Efficiency x UPS Input) = 1.5 x 1000/ (0.7*24) = 89.29A

Load Current for 1.5 hours = Answer x 1.5 = 89.29 x 1.5 = 133.94Ah

Now, it is easier for us to calculate our battery size after knowing the current usage of 1.5 hours. We need battery size of almost 134Ah.

We need two batteries which make us to use two batteries of 67Ah each to get 1.5 hours of backup. As 67Ah batteries are not available so we can use 65Ah batteries for slightly shorter backup or 70Ah batteries for slightly higher backup than 1.5 hours.



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