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Solar Design Guide: Off-Grid Battery Arrays

In the previous article in this series, we examined the fundamentals necessary for selecting an off-grid deep cycle battery. Be sure to read that article, or go back to the first article in this series for a complete overview of how to design an off-grid energy system.

Choose an Array Voltage (12 V or 24 V)

Batt	12 V	24 V
6 V	2 per	4 per
8 V	n/a	3 per
12 V	1 per	2 per

In this article, we decide what kind of battery array we are going to use. For the small solar systems described in this series (50 W to 500 W continuous 24/7 power), we only consider 12 volt or 24 volt battery arrays, although for larger off-grid systems 48 volt arrays are an appropriate choice. You will connect multiple batteries in series to make 12 volt or 24 volt strings, and then connect these strings in parallel to form the complete array. Consider the chart to the right to select the number of batteries used per string, which is simply the desired array voltage divided by the battery voltage.

Except for the smallest off-grid systems, such as one supplying only 50 watts of continuous power, we highly recommend a 24 volt battery array. As we discuss in later installments, all of the charger options we recommend can supply either 12 volt or 24 volt arrays. But, since a charger delivers its rated current to either size array, a given charger can supply twice the wattage to a 24 volt array at no additional cost. However, some readers may already have a significant amount of 12 volt devices, such as RV appliances. For these readers, a 12 volt array, and twice the number of chargers for that array, might make more sense. As a result, we show 12 volt and 24 volt options in the system diagrams used in the design guide.

Calculate the Number of Batteries

Now that you have selected a battery, an array voltage, the number of batteries per string, and the total watt-hours required, we can finally calculate the number of batteries required. First, calculate the total watt-hours per string, as follows:

Watt-hours per string = Batteries per string x Battery voltage x 20-amp-hour rate

For example, assume we are building a 24 volt array from the 6 volt, 215 amp-hour Duracell GC2 battery. This calculation is then:

Watt-hours per string = 4 x 6 volts x 215 amp-hours = 5160 watt-hours (Wh) per string

Next, calculate the number of strings required to fill the total watt-hour requirement:

Number of strings = Total watt-hours / watt-hours per string

In our example, if we are supplying 500 continuous watts, we will need 20000 Wh overall, so:

Number of strings = 20000 Wh / 5160 Wh per string = 3.876 strings

Finally, round the number of strings up to the next largest whole number, and multiply by the number of batteries per string. This is the total amount of batteries needed in your array.

Number of batteries = Number of strings (rounded up) x batteries per string

In our example, we would require, after rounding 3.876 strings up to 4 strings:

Number of batteries = 4 strings x 4 batteries per string = 16 batteries total

This example was straightforward, but you will probably want to try the math with several different battery types to see what is the best option for you. To simplify this, we have created a handy Off-Grid Battery Calculator for you to use.

When you are satisfied with your battery and array choices, it is time to select the solar panels themselves.

Next article: Selecting Solar Panels ...

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