# How to Size a Solar Charge Controller?

A solar charge controller is used to manage the power from the solar cells to charge the batteries and supply the load in the solar power system. The charge controller protects the batteries from overcharging or deep discharging, so as to guarantee the safety of the batteries, increase the service life of batteries and then improve the solar system performance.

So, really it's worth doing more investigation in your solar system for best sizing a proper solar charge controller. It is the key for longer life and higher efficiency for your battery bank as the charge controller optimizes the power and voltage coming from your solar panels to provide best charging voltage and current to the batteries.

Maximum Power Point Tracking (MPPT) Solar Charge controller

In the past, we chosen the solar array voltage to be same as the battery voltage, and made our calculation based on this. Today, with the presence of MPPT charge controller, we are able to have solar array with higher voltage than batteries voltage; because of the design of MPPT charge controller, it converts higher voltage to lower voltage.

This amazing feature enables us to install small gauge wires at solar array system side than before as when the solar array voltage is doubled, the current will be reduced to the half and then the wire gauge will be reduced accordingly, consequently the wiring cost will dramatically reduce.

Even due to the unique advantage of MMPT solar charge controller that has the capability of receiving higher voltage from solar array and handling lower rated voltage batteries, you become free of matching voltage condition, and your design become open, that is, you can use different design scheme with different material sizes as well.

How to size a solar charge controller?

This can be done through indicating 2 parameters as following:

1. Charge controller rated current

You should know that the charge controller rated current is referring to its output current not the input current coming from the solar array.

So, suppose you have solar array of 3500W and its operating voltage is 96V and you have batteries bank of 48VDC, so to indicate charge controller rated current, let us apply power equation: P = V X I. Then I = 3500/48 = 72.91A and by taking into account the possibility of solar panels to provide higher power due to sunny days or reflecting lights coming from any outer source like water or snow, we should apply 25% safety factor, then charge controller rated current = 72.91 x 1.25 = 91.13A. Consequently, we should choose/install 95A or 100A charge controller in this system.

2. Maximum solar array DC voltage

The MPPT solar charge controller has max input DC voltage limit where should be continuously larger than solar array max DC output voltage, its Not allowed to be exceeded, that is the condition.

In order to achieve that, you are needing to be sure that the open circuit voltage of the whole solar array will not go above this limit. Also, you should have a certain margin above open circuit voltage value to be in safe side as the open circuit voltage may increase due to cold weather. So, we recommend to take safety factor of 25%.

Suppose you have 12 solar modules forming 4 string (3 modules will be connected in series) and module open circuit voltage is 32V, then whole solar array open circuit voltage will be 96VDC, taking 25% safety factor, then max safety DC voltage limit will be: 120V, consequently we need to choose the controller max DC voltage limit to be 125V or 130V.