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A Guide to MPPT Charge Controller Sizing

Creation date: Nov 23, 2023 5:16pm     Last modified date: Nov 23, 2023 5:16pm   Last visit date: Jun 8, 2024 8:05pm
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Nov 23, 2023  ( 1 post )  
Wang Meihong (chenluseo)
If you're considering switching to solar power, you'll need to do lots of research before choosing your system parts and components. One of the solar devices you may consider is an MPPT charge controller. Read on to find out what an MPPT charge controller is, whether you need one, and what size you should have. 
If you're unfamiliar with solar energy, it can be overwhelming. Many components make up an optimal solar power setup, and if you're unfamiliar with electrical terms, it can seem overwhelming. When you research your ideal solar system, many questions arise. What is a solar charge controller? What is an MPPT? 
Charge controllers are one of the most challenging components of any solar power system. They are not always easy to size correctly because of their capacity and voltage. Many different factors come into play, including the capacity and voltage of both your solar array and your battery bank. There are also multiple types available, including the widely used maximum power point tracker (MPPT) controller.
This article is intended to help you decide if you should use an MPPT for your system. It is also intended to provide you with some guidance on how to find one that suits your needs.

What Is a Charge Controller

A charge controller, also known as a solar regulator, is a crucial component in off-grid and grid-tied solar power systems. Its primary function is to regulate the voltage and current from solar panels or wind turbines that charge the batteries in these systems. Charge controllers help prevent overcharging and ensure the proper charging of the batteries, ultimately extending their lifespan and improving the overall efficiency of the renewable energy system.
40a mppt charge controller

What Is a MPPT Charge Controller

A Maximum Power Point Tracking (MPPT) charge controller is a type of electronic device used in solar power systems to optimize the efficiency of energy harvesting from solar panels. The primary function of an MPPT charge controller is to dynamically adjust the electrical operating point of the solar panels, allowing them to deliver maximum power to the battery or the load. This optimization is crucial for increasing the overall efficiency of a solar power system.

What Does an MPPT Charge Controller Do

Your solar power system achieves peak efficiency when it aligns with the specifications of your batteries. Excessive power input results in significant energy loss, while insufficient input hinders the accumulation of enough energy for optimal functionality. Maintaining the right balance is crucial for your system's performance.
To maximize the wattage output from your solar system, a precise combination of amps and volts is essential. Wattage, the product of amps multiplied by volts, requires careful consideration. If your battery capacity is limited to 12 volts, the amperage must be sufficient to reach the total wattage intended for your solar panel.
Calculating this manually can be intricate. The panel must exceed the battery voltage for proper equilibrium, creating a non-linear relationship between the numbers. The maximum power point signifies the ideal balance of voltage and amperage, ensuring the most efficient energy conversion and transfer while minimizing energy loss in your system.
Moreover, the optimal levels of system amperage and voltage vary throughout the day due to factors such as the sun's angle, cloud cover, precipitation, and temperature. These variables necessitate a dynamic approach, making it challenging to determine the ideal combination through traditional calculations.
This is where a Maximum Power Point Tracking (MPPT) system becomes indispensable. The MPPT charge controller digitally monitors all relevant inputs and consistently tracks the optimum levels. It regulates the current from the panel and the voltage into the battery, continuously adjusting these levels to bring your system as close as possible to the maximum power point in each moment, ensuring the most efficient operation of your solar power system. 

What to Consider Sizing MPPT Charge Controllers

An MPPT charge controller is characterized by three primary ratings.

The first rating specifies the voltage of battery banks that the charge controller is designed to operate with.

The second rating pertains to the voltage input, a critical factor for ensuring proper functionality and avoiding damage to the charge controller. Typically, a voltage window is provided, such as between 18V and 150V for a 12V battery bank. If the solar array is undersized, falling below 18V, it may not supply sufficient voltage to effectively charge the battery. Conversely, exceeding the high voltage limit can result in permanent damage to the charge controller.

When determining the appropriate input voltage, consideration must be given to cold weather conditions. Since most solar panels are made of silicon, which experiences an increase in voltage when cold, adjustments must be made. Voltage ratings are commonly provided at Standard Test Conditions (STC), which is 25°C (77°F). In colder climates during winter, temperatures may fall considerably lower. Therefore, it is essential to account for the coldest temperature the solar panels will encounter during daylight hours.

For example, if the Voc (open-circuit voltage) of a solar panel is 38V, and there are three panels in series, and the temperature is 30°F on a cold morning, the adjusted voltage would be calculated as 38Voc x 3 in series x 1.12 = 127.68V temperature-compensated. This may be suitable for a 150V rated charge controller but exceeds the limit for a 100V controller.

The third rating relates to the output current, determined by the simple equation Watts ÷ Volts = Amps. By dividing the total watts of the solar array by the voltage of the battery bank, the output current of the charge controller is obtained. For instance, a 1000W solar array ÷ 24V battery bank = 41.6A, indicating that the charge controller should have a rating of at least 40A. Over-paneling a charge controller, i.e., introducing a higher wattage than its rating, can lead to clipping during peak output but increased output during lower-demand periods. Alternatively, undersizing the array may prolong the charge controller's life by preventing it from operating at its full capacity throughout the day.

In situations where the system output exceeds the capacity of a single charge controller, multiple controllers can be employed to manage the array. Each charge controller's output passes through its breaker in parallel to the battery bank. Higher-end charge controllers often have communication capabilities, forming an intelligent network to optimize charging. It's advisable to consult with your sales representative to determine whether rounding up or rounding down is more suitable for your specific setup.


The use of solar storage requires a charge controller, which is an integral part of your solar power system. If you are planning on living off-grid, in a tiny house, or in an RV, then solar charge controllers will be a vital part of your system. Choose the right solar charge controller that fits your space to build an efficient solar power system.

Do you have questions about a MPPT charge controller? Give an Helios solar energy expert a call today at + 86 755 2960 1174 to learn more!