The backup time is simply the number of hours, Inverter will be able to run Lighting load, during Power Failure.
The backup time in hours can be calculated using following Formula:  

Backup Time =

AH x 12V x N x P.F. x Efficiency of Battery (0.9)

Load in (VA)
        Where,      AH stands for Ampere Hour Capacity of Battery
                  N stands for Number of 12 V Batteries needed
        P.F. stands for Power Factor of Inverter
   EFF stands for Efficiency of Battery
              Load stands for Number of Tubes and Fans
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