Understanding Battery Rating, Capacity and How to Size Your Battery with Inverter

Are you planning to buy your first back up system but you don’t know how many batteries you will need that will give you the right amount of back up? This article will answer all your questions like what is battery capacity, how much back up will I get, how to size it with my existing inverter.

To understand how to find the right back up system for your home you will need to understand how batteries are rated and then how to install the right battery with your inverter.

We have made a very interested animated video on our YouTube channel to explain this technical concept. Watch now!

However, if you prefer to dig deep and read more about the subject then scroll down.


Let us begin by understanding how to read a battery capacity. Battery capacities are rated like 30AH, 100AH, 150AH, 200AH. Here A means amperes. Ampere is a unit of current. And H here means hours as in the back up time that the battery will provide. So let us take the example of a 150AH battery and find out how many hours of back up can it provide.

Battery manufacturers usually rate batteries like C1, C3, C10, C20 etc. To understand how many hours’ back up the battery can give we need to understand what these ratings mean. Most batteries that are installed with inverters are usually rated C10 in the Indian market. So let us take the example of a 150AH battery rated C10.

Here C means capacity of the battery and 10 here means charging time. Since in this example the battery capacity is 150AH we will divide 150AH by 10. Which gives us 15Amp.

This formula essentially tells us how many amperes are required to charge a battery. This means that if an inverter charges a battery with 15amp it will take 10 hours to get fully charged and also if the battery is discharged with 15amp it will give 10 hours of back up.

Similarly, if this 150AH battery is rated C20 then if you discharge the battery with 7.5amp (150AH/20) then you will get exactly 20 hours of back up from this battery. Imagine you install a 150AH C10 battery with your inverter rated at 800VA. The full load of this inverter will be 560W. And you want to calculate how much back up your battery will give you must calculate with how many amps your load will discharge the battery.

To calculate the load amp simply use the formula:


P (Power) =560W

V (Voltage)=12V (battery is 12V)

Therefore, by calculation I = 46Amps.

Your load is now discharging your battery with 46amps. That means your C10 rated battery is now reduced to a C3 battery. And at 46amps it will give just 3 hours of back up time.

Remember if you increase the discharging current then back up time reduces. For best results the discharging amp of a particular rated battery must be 10% of its AH as in if a battery is rated 150AH then it must be discharged with 15amp that will give 10 hours of back up and will maintain your battery life. Again if your battery is rated 180AH the ideal discharging current should be 18amps.


Now if you require 10 hours of back up and you install a 150AH battery you must ensure that your inverter is rated at 15amp of charging current. If your inverter is rated at 10amp charging current, then by formula your inverter will take 15 hours to charge your battery.

Now suppose you need only 10 to 15 mins back up time. In this case you will need a 30AH battery and an inverter that will provide 3amp charging current that will give you the required back up time. You need not buy a higher capacity battery in this case.

Usually inverters have a fixed charging current and therefore customers are forced to buy a battery of the size suited to the inverter. But Su-Kam has introduced Falcon+ Home UPS, India’s most intelligent smart and customer friendly UPS that has a variable charging current feature.

You can set battery charging current of 2.5amp, 5amp, 10amp and 15amp according to your back up needs. If you need 10 hours of back up from your battery, then you can install a 150AH battery with Falcon+ and set the battery charging current at 15amp. Likewise, if you want 10 to 15mins back up time you can install a 30AH battery with Falcon+ and set the battery charging current at 2.5amp thereby saving a ton of money of buying a small battery rather than a large battery.

Here’s a video to make this super clear:

Now that you know all about batteries, choose wisely. Visit www.su-kam.com to buy our products.



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