How to calculate inverter battery backup time?

How often have you been in the middle of your work when the power suddenly goes out?

Or maybe you were in the middle of watching your favorite show when the power went out, and you had to scramble to find candles.

I’m sure it’s happened to all of us at one point or another.

It can be frustrating, especially if you live in an area where power outages are common.

An inverter can help in those situations, but do you know how much backup time they provide?

This is an important question to ask if you are considering using an inverter to power your home during a power outage.

This article will show you how to calculate the inverter battery backup you need to keep your home or business running during a power outage.

Whether you are looking to be prepared for power outages or looking to buy the right capacity inverter to avoid the above issues, this guide is for you.

By the end of this article, you’ll know everything you need about inverter battery backup.

If you’re in a hurry,

Here’s a capacity vs. load comparison table to help determine how long your battery will last.

Inverter Battery backup time calculation table:

80 AH7.68 Hours4.08 Hours2.72 Hours2.04 Hours1.64 Hours1.36 Hours1.16 Hours
100 AH9.6 Hours4.8 Hours3.2 Hours2.4 Hours1.92 Hours1.6 Hours1.37 Hours
120 AH11.52 Hours5.76 Hours3.84 Hours2.88 Hours2.3 Hours1.92 Hours1.65 Hours
135 AH12.96 Hours6.48 Hours4.32 Hours3.24 Hours2.6 Hours2.16 Hours1.85 Hours
150 AH14.4 Hours7.2 Hours4.8 Hours3.6 Hours2.88 Hours2.4 Hours2.05 Hours
180 AH17.28 Hours8.64 Hours5.76 Hours4.32 Hours3.46 Hours2.88 Hours2.46 Hours
200 AH19.2 Hours9.6 Hours6.4 Hours4.8 Hours3.84 Hours3.2 Hours2.74 Hours
220 AH21.2 Hours10.56 Hours7.04 Hours5.28 Hours4.23 Hours3.52 Hours3.01 Hours
240 AH23.4 Hours11.52 Hours7.68 Hours5.76 Hours4.6 Hours3.84 Hours3.3 Hours

Remember, you need a capacity that will give you at least 3 hours of backup time at a given appliance’s watt load.


  • The above calculations assume Voltage—12v; Battery Efficiency—80%.
  • For loads above 700 Watts per hour, buying a double or triple battery is recommended to fulfill the requirement.

How long your backup battery will last depends on many factors like the capacity of the battery, its health, and the load it will carry.

So, there’s no one-size-fits-all answer.

However, the above chart can give you a quick reference to help you prepare for outages.

If you want to learn how to calculate more accurate backup time for your needs, follow the article further.

This article will help you calculate the backup time using a formula and an easy-to-use calculator.

So, let’s get started!

Table of contents:

How to calculate inverter battery backup time?

To calculate your inverter battery backup time,

You need to know the capacity, efficiency & voltage of the battery and the Watt per hour of your appliances.

But what are they? Where can you find them?

No worries, follow me along.

Battery capacity:

The battery capacity is determined by how fast the current flows and is measured in Ampere hours. 

A higher Ampere hour rating means that the electricity flows quickly and delivers more power to appliances. 

You can find the battery capacity on the battery user manual, product description, or the face of the battery itself.

Input voltage of battery:

The battery’s voltage is the unit of potentiality in your battery and is measured in volts. The higher voltage, the more current flows through the battery and power the appliances. 

In general, every home Inverter battery comes with 12v. You may find this detail in the user manual.

The efficiency of the battery:

Battery efficiency is the amount of energy you get from the battery for consumption relative to the amount of energy you input. 

Some amount of loss is unavoidable during the process of energy discharge. 

Efficiency % varies from battery to battery, which you can find in the product catalog or enquire with your manufacturer.

Generally, one can expect an efficiency of 80%, i.e., 0.8, from top battery brands like Luminous, Exide, and Amaron.

Calculate the appliance power load (Watt) requirement:

Load is the amount of power required per hour for appliances connected to an inverter and is measured in watts. 

The watt each appliance uses is usually mentioned in the appliance package, or you can find related product details on amazon.

If you can’t find the wattage for an appliance, use our load calculator to estimate how much power you’ll need.

Remember that you should only calculate the load for appliances you plan to use during a power cuts.

For example, if you want to connect only 1 fan and 2 bulbs, calculate the load requirement of that fan and bulbs. Ignore other appliances which you don’t want to use during power cuts.

After finding or calculating the above details, you can use the below formula or calculator to calculate inverter battery backup.

Inverter Battery Backup time formula:


Backup Time (in hours) = Battery Capacity (in Ah) x Input Voltage (V) x Efficiency %(n) / Total Load (In Watts)

To understand this formula, let’s assume you have an inverter battery with 150AH capacity, 12 V of input voltage, and 80% discharge Efficiency.

You have 2 tube lights of 40 watts, 2 fans of 75 watts, and 1 Wi-Fi router of 20 watts working on this battery. 

How long will it last in this case? Let’s calculate:

Let’s first figure out what the total load per hour is:

  • 2 Tube lights = 40 x 2 = 80 Watts
  • 2 Fans = 75 x 2 = 150 Watts
  • 1 Wi-Fi Router = 1 x 20 = 20 Watts

The total load, in this case, is 80+150+20 = 250 Watts.

Now, let’s put these values in our formula:

Backup Time (in hours) = 150 x 12 x 0.8 / 250 = 5.76 Hours

So, your 150 AH inverter battery is expected to provide 5.76 hours in this case.

Does the smaller or larger capacity of the battery make a difference?

Obviously, it would make a huge difference. Here are calculations of a smaller and a larger capacity of batteries:

If the capacity would be 100Ah:

Backup Time (in hours) = 100 x 12 x 0.8 / 250 = 3.84 hours

So, with a 100 AH battery, we would’ve got a backup of just 3.84 hours.

If the capacity would be 220Ah:

Backup Time (in hours) = 220 x 12 x 0.8/ 250 = 8.448

With a 220 AH battery, We would’ve got a backup of 8.448 hours, which is much more.

So, you can see that battery capacity, whether 100, 120, 135, 150, 200, 220, or 240 Ah, plays a massive role in how much backup you’ll have. 

The more capacity, the more backup time you’ll get.

But that doesn’t mean you need to purchase a higher AH battery.

Observe your battery backup time needs and choose accordingly. One should have a min 3 hours of backup time in India.

Inverter Battery Backup time calculator :


  • Calculate load requirements using this calculator.
  • Consider the above calculation as an approximation because we can’t exactly know how much an old fan or tv consumes or whether your inverter is working up to its efficiency, etc.; you can consider – a 10% to -15% buffer for the above result.

Tips to improve inverter battery backup time:

To have a long-lasting battery during power cuts, you should work focus mainly on these aspects:

Decrease the load:

By decreasing the load, you directly improve the backup time. Less load means fewer appliances, which means that your inverter can power those lesser appliances for a longer time.

If you are sitting in one room, you may only need 1 fan and 1 tube light at that particular time. So, make sure that you switch off the heavy load appliances and appliances of other rooms, so the inverter has a minimal load – Use the necessary.

Get a higher-capacity inverter battery:

If you want to power more or heavy-load appliances in your home, you would obviously want a battery with more capacity. More capacity would allow you to put a heavier load; hence, you’ll get much longer backup time.

You can play around with the calculator and check how much battery and load would affect the backup time.

Provide periodic maintenance:

Provide periodic maintenance to the inverter and battery to keep them up to maximum backup efficiency. Make sure to check the water level periodically, keep them corrosion-free, and give a frequent overall check with the help of an expert.

Use energy-saving appliances:

Look for alternatives that can save power and, at the same time, serve your purpose. This is a great way to reduce the current bill and increase backup time. 

E.g., Use LED or CFL bulbs instead of ordinary bulbs.

Change the battery when needed:

Over time, the battery’s capacity decreases, as does backup time. Consider changing the battery after 3 years, depending on manufacturers’ guidelines.


How to know the watt usage of each appliance?

When buying an appliance, the wattage is usually mentioned on the packaging. Alternatively, you can search online.

How to know your inverter battery capacity?

The battery’s capacity can be found in the user manual, product description, or on the face of the battery itself.

Capacity is expressed in amp-hours (Ah), and common values are 100 Ah, 120 Ah, 135 Ah, 150 Ah, 180 Ah, 200 Ah, 220 Ah, and 240 Ah.

Does the inverter brand calculate and provide an estimated backup time?

Yes, most good inverters will show you an estimate of how long they can provide backup power.

However, that number may not be accurate for your specific needs. Calculating the backup time based on your particular needs is always a good idea.

What minimum backup time should I look for while purchasing an inverter battery?

A minimum backup time of 3-4 hours is ideal. However, if your area experiences more frequent or longer power cuts, aim for 6 to 8 hours instead.

Which type of inverter battery is best for backup?

We recommend using tubular batteries. They are built for emergency, quick, and maximum backup. Within the tubular, go for tall tubular as they have many other benefits.

Which inverter battery is good for 8 to 10 hours of backup?

To decide that, you need to calculate your load requirements.

We recommend going for batteries above 200 Ah if you have a load requirement of fewer than 400 Watts.

For higher load requirements, you’ll need double or triple batteries. But before choosing double or triple, make sure your inverter supports it.

Which inverter battery is good for 4 to 6 hours of backup?

You’ll need to calculate your load requirements to decide that.

We recommend using 150 Ah batteries if you have a load requirement of fewer than 400 Watts.

For higher load requirements, go for >200 Ah or double or triple batteries. But make sure your inverter supports it before choosing double or triple.

How much battery backup does a Double or triple battery inverter provide?

To calculate the backup time for a multi-battery inverter, you need to multiply the number of batteries connected to the inverter by the formula illustrated in the article.

So the formula to calculate the backup time for a multi-battery inverter is as follows:

Backup Time (in hours) = [Battery Capacity (in Ah) x Input Voltage (V) x Efficiency %(n) / Total Load (In Watts)x N

Where “N” is the number of batteries.

How much battery backup does a 24 v battery inverter provide?

You need to replace 24v with 12v in the formula illustrated in this article.

So the formula to calculate a multi-battery inverter is: Backup Time (in hours) = [Battery Capacity (in Ah) x 24 v x Efficiency %(n) / Total Load (In Watts)]

Final words:

Inverter battery backup is necessary to ensure that your home is prepared for a power outage.

By understanding how to calculate your inverter battery backup, you can be sure that you have the right power to keep your home running during an outage.

Here is a quick summary of the steps to calculate your inverter battery backup time.

  • Know the battery capacity
  • Find the input voltage of the battery
  • Calculate the appliance load requirement
  • Check the efficiency of the battery
  • Calculate the backup time using the formula or calculator above.

And keep in mind that you can only calculate an estimated backup time following the above. 

There is no one-size-fit-all answer for how much battery backup time you need, 

It depends on many factors such as capacity, efficiency & voltage of the battery, and appliance load requirements.

Still, finding any difficulty? Comment below.


Hey !! This is Surendra Reddy, a blogging enthusiast. In this blog, I share reviews and informative articles on home inverters, inverter batteries. Thank you.

1 Comment
  1. Extremely useful information.
    Thank you so much

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