Solar Power: Part 6-Solar charge controller

The purpose of a solar charge controller is to regulate the current from the solar panels so that you do not damage the batteries. There are many descriptions of charge controller types and their function; I will not attempt to describe them in this article but I will refer you to this article. Why reinvent the wheel when it has been done well by others with greater knowledge.

However, what is not covered well in most other articles is how to choose the proper size charge controller. It is important to match the controller to the output of your solar panels. Getting a charge controller larger than your needs will not cause any damage but choosing a charge controller of insufficient capacity could cause a fire by overheating the controller.

Of the two basic types of charge controllers (PWT and MPPT), I much prefer the MPPT because of better charge regulation and because more of the solar panel output is directed to the batteries. Reading the above referenced article will help decipher a bit of the fog about charge controller types. I also recommend the Solar Electric Handbook (see link at bottom).

Solar controllers have a rated amperage. For example, my Renogy MPPT solar controller is rated at 40 amps. This is the maximum input amperage. Solar panels are rated in watts. We are back in the United Nations where everyone is speaking a different language (amps, watts, volts, etc). However, we can convert watts to amps with the following formula: Amps = Watts/Voltage.

In part one of this series, I stated that we are designing a basic 12V solar charge system costing $3500-5000. You can build a larger or smaller system, this just gives us a starting point so I can restrict my discussion to components that fit this system.

So, how many solar panels can I add to my solar charge system if I have a 40 amp charge controller. Forty amps is 480 watts for a 12 volt system. However, you do not want to exceed the capacity of the charge controller, when the sun is high in the sky with clear blue skies,  your panels will frequently exceed their rated capacity. So, let's leave a 10% margin of error (480 watts - 10% = 432 watts). Therefore, we can safely add 432 watts of solar panels to our system.

Solar panels are cheaper per watt the larger the panel you purchase. Typically, one 200 watt panel will be cheaper than two 100 watt panels. You will also save on mounting hardware, wires, and such by purchasing higher capacity panels. Panels also come in various sizes and it is important to match the panel size to your available space. This image shows the various components of my solar charge system (except batteries and solar panels); it is important to place the charge controller in an area that will get some circulation, charge controllers have cooling fins and need air circulation.

I will cover solar panels in Part 7 of this series. For my system, I choose four 100 watt panels because this fit the space I had available for my unit and this did not exceed the capacity of my charge controller. At a future date, I will install a second charge controller and four additional 100 watt panels.

There are many choices for charge controllers and books, these are the ones I have purchased and had good luck using. I wish you all well.