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.

Solar Power: Part 1-Introduction

Entire books are written the subject of solar setups so I will try to digest it down to the basics.  The book I would recommend for learning about setting up a solar power system is the Solar Electric Handbook. (Purchase of any of these items through our links will give us a few pennies to cover internet costs and allow us to continue providing additional content, thank you).

 Designing a solar power system can be fraught with tension and stress. There are so many terms and it seems each device uses a different nomenclature. There are watts, amps, amp hours, volts, etc. It is enough to make you think you are at the United Nations and no one has a translator.

In a series of post, I will show you how I designed my solar power system and I will detail the reasons for each item I selected. I am assuming you want a basic system that cost $5,000 or less and will provide for basic needs such as a refrigerator, LED lights, tv, fans, laptop and mobile phone charger, and other basic needs. Appliances requiring high electrical needs (heaters, large air conditioners, clothes dryers, etc) can be operated with solar but require a larger system and most likely you will need a qualified professional for the install. What I will detail is a basic system I use in my tiny house.

Let us first list the required components: solar panels, solar controller, inverter, batteries, fuses, maybe a battery charger, transfer switch and monitors, and wires to connect all together. I installed the solar controller, inverter, transfer switch and my main electrical panel in a box on the tongue of my tiny house trailer. These items are outside my home but protected from the elements. The attached photograph shows the basic components. I will detail my reasons for each item in a series of blog posts but many other items work just as well if not better.

Batteries can not be stored outside in my region because they will loose amperage in cold weather. I have four deep cycle AGM batteries installed into a cabinet that is also my lamp stand adjacent to my chair.

Your first duty is to determine: Can you afford to go solar. If all you need is one or two LED lights and charging your mobile phone, you can setup a system for $100 or so. However, operating a tiny house with more comfortable needs will cost a bit more and will depend on your needs. A basic system will cost approximately $3-4,000 dollars (assuming new components). I define a basic system as one that can power an apartment size refrigerator, tv and dvd player, laptop computer, LED lights, fans, and other basic needs.  Therefore, moving forward, we are designing a basic system as I defined above.

Wow, $4,000 dollars, that is a bit steep for a basic system-some will say. However, the cost to get electrical service in the bonnies can top well over $30,000 or more. Solar is also an investment, being off-grid frees you from the invoices of the power grid. If you are a prepper, solar will be the only power available when the grid is shutdown by unrest or other problems.

Most books and professionals will tell you first need to add up all your electrical needs to design your system. I prefer to work backwards designing my system (figure out the system I could afford and then see what it would power) because I am on a limited income. I have no choice but to do solar because the electrical grid is not available to me.

So, this is how I started. I planned for a $3500 system with all new quality components. This would be able to generate 400 watts of power from four 100 watts solar panels. This is where it get confusing; it is not complicated but can be tedious because not all items speak the same language. Your friend is the following formulas:

Watts = Amps X Voltage

or the same formula rewritten as

Amps=Watts/Voltage

I may have lost a few of you, not to worry, this will not be a math class. I will give you instructions to figure your needs. Let us assume you live up north where there is less sunshine in the winter. If you live in Texas, Florida or other areas with abundant sunshine, you will have much more power available 12 months of the year. Living up north, we get about 7 hours of usable sunshine per day in the winter (assuming blue skies or light overcast).  Remember that figure from above, 400 watts from the solar panels. Take the hours of usable sunshine per day (7) and multiply it by the 400 watts to give you the total energy generated per day in the winter from your solar panels; in this example that is 2800 watts. You will get some loss of power because of resistance of the wires and such so let us assume you are getting 2500 watts of power added to your batteries each day. Of course, in the summer you will have more sunshine but you need to design you system for the times of the year with limited sunshine. Unless you have a summer cabin that is not used in winter or some other twist; I am assuming you are designing this system for year around use.

WooHoo, now we are making progress. But wait, batteries are measured in amps or amp hours.  No worries, just convert it. 2500 watts on a 12V system is 208 amps (2500 watts/12 volts = 208 amps). So, on this one day of sunshine you have added 208 amps to your batteries (assuming the batteries needed to be charged). Wait, you can not use all that energy because your batteries should not be discharged below 50% (assuming you have lead acid batteries, not salt water batteries). Therefore, divide the 208 amps by half to get roughly 104 amps for your daily use.

Ooops, here we are again at the United Nations; most electrical items are not rated in amps, many are rated in watts. For example, a 60 watt bulb or a 37 watt refrigerator. No worries, just convert it again: 104 amps X 12 volts = 1248 watts. You have 1248 watts available for your 24 hour day; the next day more energy will be generated. Some days, you might have excess and other day, you might draw more than what is generated from the batteries.

In the next blog post in this series, I will explain batteries and how to calculate your needs for battery storage and use.