Home » Miscellaneous19 November 2012

How to Calculate Inverter & Battery Backup Time?

Here is a quick guide on calculating inverter and battery backup time.

There are different capacity of inverters like 200VA, 250VA, 350VA, 400VA, 450VA, 500VA, 625VA, 1200VA, etc. The capacity of the inverter should be chosen based on how much watt of appliances you power at a time.

How to convert from VA to Watts?

Volts x Ampere = VA (VoltAmpere)
Volts x Ampere x Power Factor = kW (Watts)

Power Factor = 0.8 (which is the max. power factor of home standard)

If you want to run 200W of appliances then,

200 / 0.8 = 250VA

So, you should prefer inverter of 250VA or more. 300VA inverter will be the best choice.

Similarly, battery is an integral part of the inverter. The performance of the inverter depends on the connected battery and vice-versa.

The battery backup time in hours can be calculated using following formula:

Backup Time = Battery AH x 12V x N x Efficiency of Battery / Load in Watts

Battery AH = Ampere Hour Capacity of Battery
N = Number of 12 V Batteries needed
Efficiency of Battery = Generally it is 0.8, which is the max. power factor of home standard

Note: The inverter capacity is not used in backup time calculation.

Suppose, I have a 60A battery. My load is 150W. I have a single 12V battery. Generally in home standard, only single 12V battery is used.

Backup Time = ( 60 x 12 x 1 x 0.8 / 150 ) hours
= 3.84 hours

Some more information:-

Guidelines for buying an inverter

How Much Power Does My Laptop Really Use? (This is interesting one)

Hope it helps.

From Mukesh Chapagain's Blog, post How to Calculate Inverter & Battery Backup Time?

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  • Bamidele Oluwafemi


  • Daniel

    Cheers Sir good mathematics.

  • Koomaresun

    Nice explanation!!

  • Rajeev

    I have two old but functional 100 AH batteries, my max electricity consumption would be 250 W. according to your calculation 312 to 416 va inverter would be sufficient (Power factor .8 to .6). My query is can this inverter charge my batteries? or should I go for higher capacity inverter? Provided both the batteries need to be used or any additional charger are available? I would highly appreciate your help