Solar Charge Controller Sizing and How to Choose One

Authors Note: This has been updated on Feb 23, 2022 with updated information, links, and resources.

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 (solar panels) and storage (batteries). Charge controllers prevent your batteries from being overcharged 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 prevent your batteries from being overcharged 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, including overload protection, low voltage disconnects, and blockage of reverse currents.

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, your system may overload. This can lead to overheating or even 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.

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 YOU ALWAYS NEED A SOLAR CHARGE CONTROLLER?

Typically, yes. You don’t need a charge controller with small 1 to 5 watt panels that you might use to charge a mobile device or to power a single light. 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.

Solar charge controllers play an integral role in solar power systems, making them safe and effective. You can’t simply connect your solar panels to a battery directly and expect it to work. Solar panels output more than their nominal voltage. For example, a 12v solar panel might put out up to 19 volts.

While a 12v battery can take up to 14 or 15 volts when charging, 19 volts is simply too much and could lead to damage from overcharging. Solar charge controllers aren’t an optional component that delivers increased efficiency. They’re an absolute necessity that makes solar power battery charging possible.


What will affect my decision-making when selecting a charge controller?

The following factors should be considered when buying a charge controller:

• Your budget

• Lifespan of the technology

• Climate where your system will be installed: Certain charge controllers operate better in colder climates.

• How many solar panels you have and how high your energy needs are

• Size, number, and type of batteries you’re using in your system

These factors all interact in complex ways that can be challenging to implement effectively. However, there is a clear process for determining which charge controller is right for your application.


Read More:

Factors to Consider When Deciding to Purchase a Charge Controller



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.

Each of the two main types has certain cases where they are the clear choice. However, there’s more to choosing a charge controller than simply choosing the right type — in addition to this, you must consider additional features in terms of both safety and convenience.

The quality of these products can vary widely, even within the two main categories.

Pulse Width Modulation Charge Controllers: Best for those with small-scale systems
Cost: $20-$60
Best for:
Those with smaller systems (vans, RVs, tiny homes), those living in warmer climates 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.

While effective, this pulse width modulation results in a loss of power between your solar panels and your batteries. This type of charge controller can’t adjust voltages, only switch off intermittently to prevent excessive voltage to the batteries.

The voltage and current put out by your solar panels are always shifting, so this inevitably leads to some waste when using a PWM solar charge controller.

When batteries are full, PWM charge controllers keep supplying a tiny amount of power to keep your batteries full. This two-stage regulation 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.


Pros:

• Cheaper than MPPT controllers

• Best for smaller systems where the efficiency is not as critical

• Best for warm sunny weather

• Typically longer lifespan due to less components that may break

• Performs best when the battery is near the full state of charge


Cons:

• Less Efficient than MPPT controllers

• Because solar panels and batteries have to have matching voltages with these controllers, they are not ideal for larger, complex systems

Maximum Power Point Tracking Controllers: Best for those wanting a highly efficient
system Cost: $100-$729
Best for:
Those with larger systems (cabins, homes, cottages), those living in colder climates

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.

As the power output of your solar panels varies with changing conditions, there will always be a particular voltage that will provide the most optimal results.

This voltage is the maximum power point that your MPPT charge controller follows. In doing so, it ensures that you maintain optimal power output every moment that your solar panel is in operation.

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.

MPPT charge controllers are highly recommended for most large solar power systems. PWM charge controllers are typically only a viable option for portable applications such as for RV trips or possibly for a small off-grid cottage.

MPPT charge controllers deliver superior performance, with the only real downside being the additional cost compared to more basic options.


Pros:

• Highly efficient

• Best for larger systems where the additional energy production is valuable

• Best in colder, cloudier environments

• Ideal for situations where the solar array voltage is higher than the battery voltage

• Performs best when the battery is in a low state of charge


Cons:

• More expensive than PWM controllers

• Typically shorter lifespan due to more components


Read More:

Charge Controller Types

What To Know About MPPT Solar Charge Controllers

What is the Difference Between MPPT and PWM Charge Controllers?



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.

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 you haven’t sized your system yet or calculated your energy needs, we recommend using the Renogy solar panel calculator. This will help you size your solar panels, as well as all of the other components in your system.

When it comes to sizing your system properly, the amps are the value you’ll have to pay the most attention to for your charge controller. You need the right voltage as well, but that’s as simple as matching the system and charge controller nominal voltages.

The amperage is instead based on your energy use and battery capacity, which can be much more challenging to determine.

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. Here’s some more specifics based on the type of charge controller you have installed in your system.

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 a 30 amp rating charge controller. 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. This tells us what batteries are compatible with the charge controller. This is important to check as you don’t want to have batteries that cannot be charged by the controller unit.

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 ability of the MPPT controller 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.

Being able to accept higher voltages makes MPPT controllers particularly suited to some specific applications. Higher voltages lead to less power loss across a length of wire, which is why long-distance transmission lines have such high voltages.

If your battery banks are some distance from your panels, running the system at higher voltage and relying on MPPT solar charge controllers is the best way to cut down transmission loss.



CAN YOU USE MORE THAN ONE CHARGE CONTROLLER?

You can use multiple charge controllers with one battery bank in situations where a single charge controller is not large enough to handle the output of your solar panel array. In fact, for MPPT charge controllers, this can be the best way to connect your system as arrays have different maximum power points. Having two controllers can optimize the total power output.

In many cases, individuals who install solar power systems will later go on to expand these systems. It isn’t uncommon for the capacity of the expansion to go well over what the existing charge controller can handle.

However, we do recommend using the same type of charge controllers if you are using more than one. So if you have one MPPT charge controller, all of your charge controllers should be MPPT. Additionally, you’ll want to make sure all your controllers have the same battery setting input.



WHAT IS THE UPPER VOLTAGE LIMIT?

All charge controllers have an upper voltage limit. This refers to the maximum amount of voltage the controllers can safely handle. Make sure you know what the upper voltage limit of your controllers is. Otherwise you may end up burning out your solar charge controller or creating other safety risks.

While there are many other factors at play to determine whether you’re choosing the right size charge controller, there is very little wiggle room when it comes to the upper voltage limit.

Choosing the wrong size in terms of amperage can leave you without the capacity you need from your charge controller, but an insufficient upper voltage limit will lead to your system not functioning at all.

You need to make sure that your charge controller is able to handle the maximum voltage that’s put out by your solar power system. In general, this is a serious concern if you’re running solar panels in series.

When connected in series, the voltage adds up with each panel. So your two 12v panels are now putting out 24v, which will surely fry your 12v charge controller.



COMMON CHARGE CONTROLLER MISTAKES AND ERRORS

Because of all the different components of a solar installation, it can be easy to make a misstep in the installation process. Here are a few commonly made mistakes when it comes to solar charge controllers.

• Do not connect AC loads to the charge controller. Only DC loads should be connected to the charge controller’s output.

• Certain low-voltage appliances must be connected directly to the battery.

• The charge controller should always be mounted close to the battery since precise measurement of the battery voltage is an important part of the functions of a solar charge controller.

During operation, there are a few potential issues that can arise with your charge controller. You could find yourself with no power at all from your solar power system. This situation could be the result of either disconnected or improperly connected wires. Check to see if all connections have been made and verify that none are reversed.

The temperature of various components is of serious concern when it comes to your charge controller. The worst-case scenario is the core of your battery overheating, which could lead to significant damage. The controller itself can also overheat, which should happen before the battery does, in order to prevent further damage.

For the highest level of safety from your solar power system, you should look into a battery temperature sensor. This simple device can monitor your battery temperature to prevent disastrous overheating.



WHAT ARE THE DIFFERENCES BETWEEN RENOGY CHARGE CONTROLLERS?

Renogy produces three main models of charge controllers: the Wanderer, Voyager, and Rover.

Wanderer Model (PWM Charge Controller)
The Wanderer models are designed for small and simpler solar systems. They can be used with many types of battery banks, including flooded, gel, sealed, or lithium iron phosphate. Both models are compatible with 12V or 24V systems.

Wanderer 10A: Can support up to 120W on a 12V or 240W on a 24V system. The controller also features integrated 5V 2A USB ports to charge USB devices, an LCD screen, and multiple LED indicators for displaying system operation information. There is no Bluetooth port, so this unit is not compatible with the optional Bluetooth module.

Wanderer 30A: Can support up to 400W on 12V systems. The Wanderer 30A does not have built-in USB ports or an LCD screen, but offers multiple LED indicators for displaying system operation information. This model also features a bluetooth port.


Voyager Model (PWM Charge Controller)
The Voyager is Renogy’s only waterproof charge controller and is perfect for outdoor applications.

Voyager 20A: Can support up to 240W on a 12V system. It features an LCD screen and multiple LED indicators for displaying system operation information, is compatible with seven different battery types, including lithium ion, lithium iron phosphate, LTO, gel, AGM, flooded, and calcium, on a 12V system, and offers 5-stage charging. There is no bluetooth port on the Voyager.

An optional temperature sensor is also available to monitor the temperature at the battery. When the battery is located a moderate distance away from the charge controller, a temperature sensor is highly recommended.

Rover Model (MPPT Charge Controller)
The Rover was designed for the most efficient and advanced solar power system. It can be used with flooded, gel, sealed, or lithium iron phosphate batteries. The 20A, 30A, and 40A models are compatible with 12V or 24V systems. The 60A and 100A models can support 36V or 48V systems. Each of the Rover models has an LCD screen and multiple LED indicators, customizable parameters, and error codes, as well as 4-stage charging, and temperature compensation to increase your battery life and improve your system's performance. All of the Rovers also have a Bluetooth port.

Rover 20A: Can support up to 260W on 12V or 520W on 24V systems.

Rover 30A: Can support up to 400W on 12V or 800W on 24V systems.

Rover 40A: Can support up to 520W on 12V or 1040W on 24V systems.

Rover 60A: Can support up to 800W on 12V, 1600W on 24V, 2400W on 36V, or 3200W on 48V systems.

Rover 100A: Can support up to 1300W on 12V, 2600 watts on 24V, 3900 watts on 36V, or 5200 watts on 48V systems.



HOW CAN YOU REMOTELY MONITOR MY CHARGE CONTROLLER?

As mentioned above, certain charge controller models have LCD screens and LED indicators for monitoring the system from the unit. If you’d like to remotely monitor your system from wherever you may be, you’re in luck. Remotely monitoring your charge controller has never been easier, thanks to the data module for Renogy charge controllers. The Renogy DM-1 4G LTE module is capable of connecting to select Renogy charge controllers through an RS232 port and can be paired with Renogy's 4G monitoring app.

Available for smartphones and tablets, the app allows you to conveniently monitor your system and change system parameters remotely using your device anywhere the 4G LTE network service is available. Renogy’s Data Module is available with a year of prepaid service powered by T-Mobile’s 4G LTE network. You may also opt to purchase your module without prepaid service and add it to an existing cell phone plan through your mobile provider. The app is currently only available on T-Mobile and AT&T networks.

In addition to LTE options, Renogy also provides a selection of Bluetooth modules for short-range remote monitoring. These simple and affordable modules allow you to monitor the conditions of your charge controller when in use at your home, cottage, RV, or another off-grid site. The various models can offer a range of up to 82 feet.

These Bluetooth devices are powered directly through your charge controller’s RJ12 or RS485 communication port. Monitoring couldn’t be more straightforward when you can easily connect your charge controller to our Renogy DC Home app.

Our Bluetooth modules can be configured to work with a variety of solar components beyond your solar charge controller. With these convenient modules, you can monitor and control smart lithium batteries, pure sine wave inverters, battery chargers, and more.



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 the controller that’s right for you and your system.