How to calculate your UPS/inverter Battery Back-up Time?

Sometimes some petty things makes big confusion in our mind, One of those petty thing is calculating our inverter battery Back-up time.

battery-backup-time99% of people who own Home inverter would ask this question to their battery dealers at least once in their life time. But most battery shop owners don’t let their customers know the simple formula to calculate the Back-up time of inverter battery. Don’t worry readers, now I will let you know that simple formula!


Formula to Calculate the Back-up Time of Inverter Battery

Back up Time of Inverter Battery = Battery Volt x Battery AH rating / Total watts on Load



If a Person use 1 ceiling Fan + 1 Tube Light + 2 (15watts) CFL simultaneously with 150 AH battery, then the backup time will be calculated as

1 ceiling Fan = 75 watts

1 Tube light = 40 watts

2x 15watts CFL= 30 watts

Total = 145 watts

Back-up time of 150 AH Battery = 12 * 150 / 145

= 12.41 hours (approximately)

This is Not Accurate Why?

This calculation shows only approximate value because there will be some loss of energy when converting 12 v battery power to 220 volts through inverter.

Moreover, we cannot figure out the exact power consumption of ceiling fan, as it has speed adjustment dimmer switch. The power consumption will be low when the fan runs at slow speed.

Note :( Power consumption of old fans with big sized Regulator remains same at all speed points)


  1. senthil murugan
  2. Tony
  3. sakthi velu
    • Nasir Ahmad

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