Free Solar Battery Calculator: Calculate Fast & Easy The Solar Battery Bank Capacity And The Number Of Batteries In Series Or Parallel

We bring to your attention the following two free solar battery calculators:

A free calculator for sizing the solar battery or solar battery bank of your off-grid solar power system
A free calculator for determining the number of batteries in series and parallel in the battery bank.

These solar battery calculators help you design your solar battery or solar battery bank not only fast and easy but also cost-effectively by implementing the best design practices for achieving the optimal trade-off between solar battery size, cost, runtime, and long life.

We highly encourage you to read the quick guides of both calculators before using them.

You can find these guides below, just scroll down this webpage.

What is more, by reading these guides, you can discover valuable information that could help you improve your initial battery bank design.

In addition, you can get acquainted with our free ultimate guide to solar batteries before using our free calculators as well.

Disclaimer: Provided solar battery calculators are for informational and educational purposes only. By using these calculators you acknowledge that we can’t be held responsible for any damages as a result of using these calculators.

Table of Contents

Calculator for Sizing the Capacity of the Solar Battery Bank

*Days of Authonomy (DoA) is the number of days you need the system to operate when there is no power produced by the solar panels.

**Maximum short-term battery load is the approximated wattage that the battery is recommended to handle within a very short period, e.g. a couple of minutes. However, in the range of several seconds, the battery might sustain higher loads. This is the so-called surge load. Please always consult with the documentation of the battery vendor to get the correct value of this parameter.

Calculator for Defining the Number of Solar Batteries Connected in Series or Parallel in the Solar Battery Bank

*It is not recommended to connect more than 4 strings in parallel. Instead, increase the standalone battery capacity to decrease the number of paralleled strings. Please have in mind that
– Adding standalone batteries in series in a string increases the battery bank voltage, however, the capacity remains the same.

-adding standalone batteries or strings in parallel increases the battery bank capacity while keeping the voltage the same.

Other useful solar power calculators:

The Quick Guide To Using The Calculator For Sizing The Solar Battery Bank Of Your Off-Grid Solar Panel System

Here is the quick guide on how to use the calculator.

Input fields:

These are colored in yellow.

1. Enter your daily energy consumption in Wh or kWh – this is the total amount of energy you consume per day, based on the appliances you use in your household or RV. You can get the daily consumption of a device (in Watts, W) by multiplying the power rating (in Watts) of that device by the time of its everyday use (in hours). Eventually, you get your total daily consumption by adding the daily consumption of all the devices you use daily.

2. Enter the Days of Authonomy – Days of Authonomy (DoA) is the number of days you need the system to operate when there is no power produced by the solar panels. Typically it can be 1, 2 or 3 days.

3. Select the battery voltage, V – here you are supposed to select a value from the drop-down menu; battery voltage is a standard value that can be typically 6, 12, 24 or 48 volts.

Please select here the solar battery voltage of the whole solar battery bank and not one of the standalone batteries.

Usually, in off-grid solar power systems, the voltage of the battery bank is equal to the nominal voltage of the solar panels or solar panel array.

Later on, by using our second battery calculator, you could define the number of solar batteries connected in series and parallel if you are using the solar batteries of low voltage to build the battery bank.

By using the very same solar battery calculator you can define as well the number of solar batteries connected in parallel if your solar battery bank is composed of solar batteries of voltage equal to solar panel nominal voltage, i.e solar power system voltage.

Please have in mind that some MPPT solar charge controllers allow down-converting of solar array voltage to the next standardized lower voltage.

For example, you may have a 24V solar array and a 12V solar battery bank. In this case, please select 12V for the voltage of the solar battery.

You can find similar examples in our article devoted to the pros and cons of mixing solar panels of different wattage or learn more about MPPT charge controllers in our free “Definitive Guide to Solar Charge Controllers”

4. Select the battery type – the most commonly used battery types in solar power systems are:

Here you should select the battery type by a drop-down menu.

For more information about the various battery types, please, refer to our free ultimate guide to solar batteries or download our free book ‘Solar Power Demystified: The Beginners Guide To Solar Power, Energy Independence And Lower Bills’

Does the system has an inverter? – if you are about to power DC-devices only, your system will be inverter-less, and here you should select ‘No’. Otherwise, you need an inverter converting the battery DC power into AC power, and the option is ‘Yes’.

The solar battery calculator applies the best practices for using the depth of discharge/DoD/ of different types of solar batteries, thus ensuring the optimal compromise between the size of the battery bank and the desired long life of the batteries while taking into account their type.

Namely, 50% depth of discharge for Flooded or sealed lead-acid, Gel, AGM (Absorbed Glass Matt) and 80% DoD for Lithium batteries. As you know 0% DoD equals to 100% full battery, 100% DoD= empty battery.

Output fields

These are colored in blue.

Total battery capacity needed, Ah – the calculated battery capacity you need what as a result of the above data entered.

The total energy that could be stored in the solar battery /E/ in Wh or kWh could be calculated as follows:

E[Wh]=Battery Voltage[V]x Total battery capacity needed[Ah].

For example, you have calculated that the total battery capacity needed is 500Ah for a 12V solar battery.

So, the total energy stored in the solar battery would be:

E=12×500= 6000Wh=6kWh

Maximum continuous battery load, W – the approximated recommended nominal total wattage your battery can support for a more extended period – that is, during the day. The Maximum continuous load depends on the battery type and its capacity. Adhering to this recommended value will ensure the long life of the battery. Please consults with the battery documentation for its exact value.

Maximum short-term battery load, W – the approximated recommended wattage the battery should handle within a very short period, e.g. a couple of minutes. However, in the range of several seconds, the battery can also sustain higher loads. This is the so-called “surge” load of the battery. Please, always consult with the documentation of the battery vendor about both values.

The Quick Guide to Using the Solar Battery Bank Calculator For Defining The Number of Solar Batteries Connected in Series or Parallel

Here is a quick guide on how to use the calculator.

Input fields:

These are colored in yellow.

Select the battery bank voltage, V – the solar battery bank voltage is the system voltage you have selected for your system. Here, you are supposed to choose from a list of standard values.

Enter the battery bank capacity, Ah – this is the capacity (in Ah) you have already calculated by using our ‘Calculator for sizing the solar battery bank’ or you know it in advance.

Select the standalone battery voltage, V – ‘standalone’ means a single battery. Certainly, your battery bank can comprise more than one standalone battery.

Here, you are expected to select among a list of standard values typically used in solar power systems: 6, 12, 24 or 48 volts. This is the voltage of the specific battery model you are about to select for your PV system.

Enter the standalone battery capacity, Ah – this is the capacity of the specific battery model you are about to select for your PV system.

Output fields

These are colored in blue.

Number of batteries in series – here, you get calculated the number of batteries of the specific model you should connect in series to get the total battery capacity you need. The total battery capacity is calculated based on your daily energy needs.

Number of battery strings in parallel – we do not recommend connecting more than 4 strings in parallel.

Instead, to decrease the number of paralleled strings, you’d better select a standalone battery of higher capacity or connect several high capacity low voltage batteries in series to avoid parallel connection at all.

For optimal performance and long life, the recommended number of battery strings in parallel is up to two. Increasing their number up to four is a compromise.

Why is that?

Usually, even the very same solar batteries have a small difference in chemical processes, which in turn leads to a small difference in their voltage and internal resistance.

So, the higher voltage battery connected in parallel will discharge itself via a lower voltage battery.

In case of a faulty battery in a string, the defective battery will act as a load to the adjacent string, thus reducing its capacity over time.

The best practice is to use the batteries from the same vendor of the same chemistry, voltage, and capacity while connecting them at the same time in series and parallel.

Although it is possible to connect in parallel two batteries of the same chemistry and voltage but having different capacities, it is recommended to stick with the above mentioned general rule.

Above all, however, you should keep in mind that:

For batteries in a string (i.e., in series), adding batteries increases the voltage while the capacity remains the same.
For batteries connected in parallel, adding batteries increases the capacity and the runtime, while the voltage stays the same.

Please don’t mix batteries of different capacities, technology(chemistry) or vendors. Otherwise, you will compromise their life.

The best practice is to use the batteries from the same vendor of the same chemistry, voltage, and capacity while connecting them at the same time in series and parallel.

Number of batteries required – this is the total number of batteries you need based on the last two above calculated numbers: the total number of batteries is a multiple of the batteries in series and batteries in parallel.

Total capacity of the installation, Ah – this is the total capacity you get by connecting the above-calculated number of the selected battery type.

Please have in mind that the number of strings in parallel are rounded up to the next higher digit. This is needed to ensure that the desired battery capacity you have previously entered in the field “Enter the battery bank capacity, Ah” will always be achieved. That’s why the calculated value sometimes could be higher than the initial battery capacity entered.

Lacho Pop, Master of Science in Engineering

Lacho Pop, MSE, holds a Master's Degree in Electronics and Automatics. He has more than 15 years of experience in the design and implementation of various sophisticated electronic, solar power, and telecommunication systems. He authored and co-authored several practical solar books in the field of solar power and solar photovoltaics. All the books were well-received by the public. You can discover more about his bestselling solar books on Amazon on his profile page here: Lacho Pop, MSE Profile

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