How to Charge Controller Sizing

The array size and the load are the most important aspects of selecting the appropriate charge controller.

How to Charge Controller Sizing

The charge controller is a regulator which limits the rate of current that goes to and from the battery pack. Charge controllers are essential to prevent overcharging or completely draining a battery. Such action can reduce battery performance and the lifespan of a battery dramatically.

There are various types of charge controllers. They differ according to their sizes, displays, features and the way in which they regulate voltages. This short article focuses on the mathematical selection criteria of a charge controller based on the production capacity of solar panel and the total DC load on the system.

To select an appropriate charge controller, we need to calculate the Controller Input Current and Controller Load Current data.

Let’s assume that we are using twelve Schuco MPE220PS09 solar panels to power our off-grid cabin. The solar panels are connected in two sets in parallels and six in series. 

Calculating the Controller Array Current:

The Schuco 220 W solar panel has a short circuit current of 8.12 Amp.

Module Short Circuit Current x Modules in parallel x Safety Factor = Array Short Circuit Current

8.12 Amp x 2 x 1.25 = 20.3 Amp (minimum Controller input current)

This is the input current that comes from the solar array. The number of parallel strings in the array increases the current. To be on the safe side, it is advised to multiply the result by a safety factor of 1.25.

Calculating the Controller Load Current

Total DC Connected Watts / DC System Voltage = Max. DC Load Current

780 W / 12 V = 65 Amp (minimum Controller output current)

This is the output current that is pulled from the batteries through the controller. It is calculated via dividing the total connected DC power by the DC system voltage. The total connected DC power is the total power that all equipment that would run on simultaneously.

[image credit: solarbiz]


Quick question on charge controller selection pertaining to GEL vs. AGM batteries. I see that charging requirements are different for sealed and wet batteries; however are there also differing charging requirements between the two sealed types (GEL and AGM) that you know of? Can the same type of charge controller be used for GEL and AGM? thanks.

Thank you for the great question!

Most of the charge controllers can support the two types of sealed batteries. The Gel battery is fully charged at 13.8-14.1 V voltage range, whereas an AGM battery is charged at 14.4- 14.6V voltage range. The voltage range could be configured is most of the charge controllers, you might have to have a look at the User's Manual.

What type of charge controller do you use?

For further information on batteries, please have a look at the following article:

Where did the figure "780" for total DC connected watts come from? These are 12x220W=2640W.