What is the size of panel for charging 150 AH/24 volts battery?

It is very important sizing the panel matching the specifications of the battery. Otherwise, the battery will not perform to its maximum extent or the battery may throw the back current to the panels, hampering the performance of the panels.

​​In this post, we will specifically size the solar panels for charging the 150 AH/ 24 volts battery.

Assumption 1

It is always better to charge the battery in 6 to 8 hours rather than charging in a short period of time. This increases the life and performance of the battery.
In this example, we assume that we want to charge the battery in 6 hours. As the battery is 24 volts, therefore the nominal voltage from the solar panels must be at least 24 volts to let the current flow from panels to the battery (A voltage lower than 24 volts will not make current to flow to the battery. The current flows from higher potential to the lower potential)

Assumption 2

​The depth of discharge of the battery is 80% that is 20% current will always remain in the battery. So, we need to provide 80% of 150AH = 120AH from the solar panels.

Assumption 3

​The battery is 90% efficient that it needs 120AH/0.9 = 133.33 AH from the solar panels to produce 120AH. As I need to charge in 6 hours, therefore the panels should provide at least 133.33 AH/6 = 22.22 amperes current every hour to charge the battery in 6 hours.
Now, we need to design and size the panels in such a way that they are at least 24 volts of potential are able to provide 22.22 amperes of current every hour.
The electrical characteristics of 100 watt/ 12 volts solar panel are as follows:

Assumption 4

However, there are many losses associated with the solar panel which limits its current like:

• Dirt derating factor
• Temperature derating factor
• Transmission efficiency of the system
• Manufacturer’s tolerance derating factor

After discounting all these factors, let us assume that the maximum current from the 100-watt solar panel reduces to 4.5 Amp

Designing and sizing the solar array

​For making the system voltage from the panels equal to the desired voltage of 24 volts, connect two 100 watts/12 volts panels in series.

• The simple equation is Number of modules in series = 24 volts/12 volts = 2
• Similarly, the number of modules in parallel: 22.22 amperes/4.5 amperes = 4.9 (Round off to the nearest whole number which is 5)
• The size of the solar which charges the battery in 6 hours is

100 watts x 2 x 5 = 1000 watt
The array can be arranged in the following manner:

Conclusion

Many a times the output current from the array of the solar panels does not matches with the required current, then try to find the nearest best possible configuration to the charge the battery in the required time (A +- 10% difference between the output current from the solar panels and the required current by the batteries is acceptable).