UPS Backup time calculation formula: There are various ways to calculate the backup time and required battery capacity. Here is the most reliable one for UPS Backup time calculation formula.

Required Battery AH= Capacity Factor x Maximum Discharge Current

Maximum Discharge Current (MDC) = {KVA (UPS rating) x 1000 x  Load Power Factor}/ (ECV x Inverter Efficiency  x  No of cells)

Example: Let’s calculate the required battery capacity for a 10 kVA UPS for 30 minutes backup. Here the UPS contains 20 block batteries.

Solution: Here we need to calculate the Maximum Discharge Current (MDC) first.

MDC= {10 x 1000 x 0.8}/(1.8 x .95 x 120)=8000/205.2=38.9864

Capacity Factor for 30 minutes backup =  0.91, if we select ECV=1.8

Therefore, Required Battery AH= 0.91 x 38.9864= 35.48 AH

Now we need to select the nearest capacity battery for the desired backup.

TABLE: CAPACITY FACTOR

Final Volt

V/Cell (ECV)

 1 Min 5 Min 15 Min 30 Min 1 Hour
1.8 0.26 0.31 0.56 0.91 1.56
1.7 0.2 0.28 0.53 0.87 1.49
1.65 0.17 0.26 0.5 0.83 1.45
1.6 0.15 0.25 0.48 0.8 1.43