Sizing the right solar charge controller.
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How To Properly Size a Solar Charge Controller.

Are you trying to learn how to properly size a solar charge controller to fit your needs? Is it best to look at a solar charge controller’s watt limit or its amperage output? How does panel voltage come into play? Keep reading for an in-depth look at how to match a properly sized solar charge controller to your solar array.

Types of Solar Charge Controllers

There are two main types of solar charge controllers that are common for today’s systems. They are the Pulse Width Modulation(PWM) controller and the Maximum Power Point Tracking (MPPT) controller. There are a few differences between the two that would make one more desirable than the other. Some of those differences include cost, panel wiring flexibility, and panel sizing. Let’s take a look at each type individually.

PWM Solar Charge Controller

If cost is your only factor in considering the proper solar charge controller for your system, then a PWM solar charge controller is the way to go. The functionality is just like the name applies. Voltage is generated from the solar panel. When that voltage exceeds the battery voltage the charge controller powers on. Power is pulsed on and off in order to obtain the voltage to charge the battery bank. The PWM charge controller continues to pulse, resulting in a lower panel voltage to meet battery voltage requirements. When the bank is full, the charge controller will stop sending power to the battery bank.

Due to their simple functionality, the PWM solar charge controller is simple to manufacture out of low-cost parts. Because of the reduced cost, many basic solar kits come with them sized for the kit. Once the size of your solar array increases beyond 600 watts or uses panels with a maximum power point voltage six volts over the nominal battery voltage, it may be best to consider an MPPT solar charge controller.

MPPT Solar Charge Controller

There are several benefits of choosing to use an MPPT solar charge controller. Efficiency and versatility are two of the biggest. An MPPT solar charge controller will analyze a panel or string of panels and determine the point that the panel can produce the most watts. Because the MPPT solar charge controller is similar to a DC to DC transformer, it will then take that current and transform it into the voltage to charge the battery. This means that the panel can work at its peak output and achieve maximum harvest.

Versatility comes in to play because larger, higher voltage solar panels may be options to harvest the sun. Because of the MPPT solar charge controller’s ability to handle higher voltages, the charge controller will be able to start earlier in the day, make better use of cloudy, low light, or rainy day harvests, as well as taking advantage of more panel sizes and configurations. No longer is someone confined to using a 36 cell panel for a 12v system, allowing for the utilization of used residential panels.

Sizing the Solar Charge Controller to Match Your Battery Bank

When choosing a solar charge controller, you want to be able to ensure that it will be a good match for the battery bank. Some considerations are battery chemistry, voltage, and charging modes. Knowing the maximum charge rate of your battery bank is also important when choosing a solar charge controller. It will help to ensure that you will not overcharge a battery or bank. Some solar charge controllers will allow you to reduce the current output. This may be a handy feature if there is an intention to increase the battery bank size or change bank chemistries in a later upgrade.

Battery Chemistry Effects on Solar Charge Controller Selection

Different battery chemistry utilized for battery banks may play a role in the choice of a solar charge controller. Each of the different chemistries has different charge requirements. Therefore the solar charge controllers will have different profiles in order to best charge each chemistry. Many charge controllers will have default settings based on each of the different chemistries and a custom setting that will allow for specific settings. Installers should always reference the specific battery manufacture specifications for charging rates.

Sizing the Solar Charge Controller to match your panels.

The solar panel specifications need to be evaluated to properly size your solar charge controller. Other information that you need to identify is the size of the solar array and wiring plans. Decisions like the solar array size will help to identify the number of controllers that you may require. If you have not read my Blog post on Sizing a Solar Panel Array to Fit Your Needs, take a few minutes to get an understanding of the importance of being able to layout and wire the solar panel array.

Wiring Diagram Twin 100/50 solar charge controllers.
Twin Victron 100/50 Solar Charge Controllers

Because the cost of solar panels has dropped, it is a common practice to over-panel solar charge controllers by as much as 30%. What this means is that when looking at the total wattage that a charge controller is designed for, add an additional number of watts of potential panel wattage. A Victron 100/30 MPPT solar charge controller is designed to convert 440 watts of solar panels at 12 volts nominal to output 30 amps of power going to the battery bank. When panels are not pointing towards the sun, panel efficiency may see a drop by over 30%. This charge controller is perfectly capable of handling three 36 cell 200-watt panels in series.

Physical Size and Cost For Solar Charge Controllers

In the RV world, a lot of decisions end up coming down to size and cost. It is no different when trying to properly size the solar charge controller. Components take up space and in an RV that is a very valuable commodity. When considering solar charge controllers, be sure to evaluate their physical dimensions for size. Some of the lower-cost controllers are dimensionally larger. At the same time, the cost per watt managed for the charge controllers is not always a cheaper the bigger approach. For example, two Victron 100/50 MPPT solar charge controllers are dimensionally smaller and cheaper than a 150/100 MPPT solar charge controller.

Charge controllers are not always cheaper per watt managed as the size goes up.
Size Matters

Still looking for some assistance?

If you would like a consultation to discuss your specific needs, send me a message at Justin@OptingOutofNormal.com. We can get a meeting scheduled. I do have a list of recommended vendors that do pay an affiliate fee. I could get credit based on people following links in my blogs or working with those vendors to quote packages for consultation projects. If you would like to source your own parts and equipment, consulting fees may apply depending on complexity and involvement. 

A complete list of parts and equipment that we utilize in our system can be seen at Kit.co/optingoutofnormal

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Check out our blog How to size a battery bank to fit your needs. We show how we get the most benefit out of our Battle Born battery setup. If you are boondocking in the National Forest or plugged into an outlet, you can harness the great features of the batteries. This allows you to maintain your energy loads while still living normally in an RV.

Check out our blog How to size an inverter to fit your needs. I will show you how we have come up with our inverter sizing to fit our needs. I will also show our system wiring.

Check out some of our other blogs to see how we use our system. 

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Are you in search of a like-minded group of solar enthusiasts? Maybe you are looking to find the latest tips and tricks for your install? Come check out my group on Facebook. 

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