Solar Charge Controller Sizing and How to Choose One

Solar charge controllers are a critical component in every solar installation. They protect your battery storage components, and they ensure everything runs efficiently and safely throughout the lifespan of your system.

What are solar charge controllers?

The charge controller in your solar installation sits between the energy source (panels) and storage (batteries). It prevents overcharging of batteries by limiting the amount and rate of charge to your batteries. They also prevent battery drainage by shutting down the system if stored power falls below 50 percent capacity and charge the batteries at the correct voltage level. This helps preserve the life and health of the batteries.

How do solar charge controllers work?

In most charge controllers, a charge current passes through a semiconductor which acts like a valve to control the current. Charge controllers also prevents overcharging of batteries by reducing the flow of energy to the battery once it reaches a specific voltage. Overcharging batteries can be particularly damaging to the battery itself so charge controllers are especially crucial.

Charge controllers also offer some other important functions:

Overload protection: Charge controllers provide the important function of overload protection.If the current flowing into your batteries is much higher than what the circuit can deal with, you system may overload. This can lead to overheating or possibly cause fires. Charge controllers prevent these overloads from occurring. In larger systems, we also recommend a double safety protection with circuit breakers or fuses.

Low voltage disconnects: This works as an automatic disconnect of non-critical loads from the battery when the voltage falls below a defined threshold. It will automatically reconnect to the battery when it is being charged. This will prevent an over-discharge and protects equipment from operating at dangerously low voltages.

Block Reverse Currents: Solar panels pump current through your battery in one direction. At night, panels may naturally pass some of that current in the reverse direction. This can cause a slight discharge from the battery. Charge controllers prevent this from happening by acting as a valve.

Do I always need a solar charge controller?

Typically, yes. You don’t need a charge controller with small 1 to 5 watt panels. If a panel puts out 2 watts or less for each 50 battery amp hours, you probably don’t need a charge controller. Anything beyond that and you do.

Different types of solar charge controllers

There are two main types of charge controllers to consider: the cheaper, but less efficient Pulse Width Modulation (PWM) charge controllers and the highly efficient Maximum Power Point Tracking (MPPT) charge controllers. Both technologies are used widely, protect the battery, and typically have a lifespan of around 15 years, although that may vary from product to product.

Pulse Width Modulation Charge Controllers: Best for those with small-scale systems
Cost: $20-$60

Pulse Width Modulation charge controllers have been around longer and are simpler and less expensive than MPPT controllers. PWM controllers regulate the flow of energy to the battery by reducing the current gradually, called "pulse width modulation.” In contrast to providing a steady output, pulse width modulation charge controllers provide a series of short charging pulses to the battery.

When batteries are full, PWM charge controllers keep supplying a tiny amount of power to keep batteries full. This two-stage regulating is the perfect fit for a system that may experience little energy use. PWM controllers are best for small scale applications because the solar panel system and batteries must have matching voltages. The current is drawn out of the panel at just above the battery voltage.

Many PWM charge controllers come with a diverse set of extra features. Renogy’s Wanderer 10A PWM charge controller can be used with a 12V or 24V battery or battery bank and comes equipped with self-diagnostics and electronic protection functions to prevent damage from installation mistakes or system faults.




Maximum Power Point Tracking Controllers: Best for those wanting a highly efficient system
Cost: $100-$729

Maximum Power Point Tracking charge controllers are efficient at using the full power of your solar panels to charge your batteries. With MPPT controllers, the current is drawn out of the panel at the maximum power voltage, but they also limit their output to ensure batteries don’t get overcharged. MPPT charge controllers will monitor and adjust their input to regulate the current from your solar system. The overall power output will increase as a result and you can expect efficiency ratings of 90% or higher.

For example, if it becomes cloudy, your MPPT charge controller will decrease the amount of current drawn in order to maintain a desirable voltage at the output of the panel. When it becomes sunny again, the MPPT controller will allow more current from the solar panel once again.

How to size your charge controller

When it comes to charge controller sizing, you have to take into consideration whether you’re using a PWM or MPPT controller. An improperly selected charge controller may result in up to a 50% loss of the solar generated power.

Overall though, sizing charge controllers is fairly straightforward. Charge controllers are sized depending on your solar array's current and the solar system’s voltage. You typically want to make sure you have a charge controller that is large enough to handle the amount of power and current produced by your panels.

Typically, charge controllers come in 12, 24 and 48 volts. Amperage ratings can be between one and 60 amps and voltage ratings from six to 60 volts.

If your solar system's volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps. However due to environmental factors, you need to factor in an additional 25% bringing the minimum amps that this charger controller must have to 17.5 amps. So in this case, you would need a 12 volt, 20 amp charge controller.


PWM Charge Controller Sizing: PWM controllers are unable to limit their current output. They simply use the array current. Therefore, if the solar array can produce 40 amps of current and the charge controller you’re using is only rated to 30 amps, then the controller could be damaged. It’s crucial to ensure your charge controller is matched, compatible with, and properly sized for your panels.

When looking at a charge controller, there are a range of things to examine on its list of specifications or label. A PWM controller will have an amp reading for it, for example 30 amp PWM controller. This represents how many amps the controller can handle, in the case above, 30 amps. Generally the two things you want to look at in a PWM controller is the amperage and voltage rating.

Firstly, we want to look at the nominal system voltage. This will tell us what voltage battery banks the controller is compatible with. In this case, you can use 12V or 24V battery banks. Anything higher, such as a 48V battery bank, the controller will not be able to work on.

Secondly, we look at the rated battery current. Let’s say in this example you have 30 Amp rating. We recommended a factor of safety of at least 1.25, meaning you would multiply the current from your panels by 1.25 and then compare that to the 30 amps. For example, five 100 Watt panels in parallel would be 5.29 x 5 = 26.45 Amps. 26.45 Amps x 1.25 = 33 amps and would be too much for the controller. This is because the panel can experience more current than what it is rated for when exposure to sun rays is above 1000 Watts/m^2 or tilted.

Thirdly, we can look at the maximum solar input. This tells you how many volts you can have going into the controller. This controller cannot accept more than 50 volts in. Let’s look at having 2 x 100 Watt panels in series for a total of 22.5V (open-circuit voltage) x 2 = 45 volts. In this case, it will be ok to wire these two panels in series.

Fourthly, we can look at the terminals. Each controller will usually have a maximum gauge size for the terminal. This is important when purchasing wiring for your system.

Finally, look at battery type. These tells us what batteries are compatible with the charge controller. This is important to check as you don’t want to have batteries than cannot be charged by the controller unit. If you want to minimize the power losses with a PWM charge controller, you should always connect a solar panel with maximum power point voltage closer to the battery bank’s voltage.


MPPT Charge Controller Sizing: Because MPPT controllers limit their output, you can make an array as large as you want and a controller will limit that output. However, this means your system isn’t as efficient as it could be since you have panels that aren’t being properly utilized. MPPT controllers will have an amp reading for it, for example a 40 Amp MPPT Controller. Even if your panels have the potential to produce 80A of current, an MPPT charge controller will only produce 40A of current, no matter what.

MPPT controllers will have an amp reading for it, for example a 40 Amp MPPT controller. They will also have a voltage rating, but unlike PWM the input voltage rating is much higher than the battery banks it will charge. This is due to the special property of the MPPT controller being able to lower the voltage to the battery bank voltage and then increase the current to make up for lost power. You do not have to utilize the high input voltage if you want to avoid series connections in small systems, but it is very beneficial in larger systems.

Let’s say a controller’s label shows that it can handle 12V or 24V battery banks. Look for the Rov value. For example, if it is Rov-40, this means it is rated for 40 amps of current.

Thirdly, we can look at the maximum solar input voltage. For example, if an MPPT Controller can accept 100 volts of input, it will then take this (up to) 100 volts and step it down to your 12V or 24V battery. Let’s say you have 4 x 100 Watt panels in series, each with an open-circuit voltage of 22.5V. Those 4 in series will be 4 x 22.5 V = 90 Volts, which the controller can accept.

Conclusion

Whether you’re in an RV or an off-grid cabin, charge controllers are an essential part of your solar installation. Doing the research and weighing your options before making that investment will ensure you select selecting the right controller that’s right for you and your system.