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Beginner’s Solar Charge Controller Introduction

What is a Solar Charge Controller? A solar charge controller is a crucial component in a solar power system, especially in off-grid or hybrid systems where batteries are used to...

What is a Solar Charge Controller?

A solar charge controller is a crucial component in a solar power system, especially in off-grid or hybrid systems where batteries are used to store energy generated by solar panels. Its primary function is to regulate the voltage and current from solar panels going into the battery bank to prevent overcharging and over-discharging, thereby extending the lifespan of the batteries and ensuring efficient charging.

Modern solar charge controllers have evolved significantly to offer more precise and efficient charging solutions, as well as additional features to enhance the functionality of solar power systems.
Smaller solar charge controllers with capacities ranging from 12V to 24V and up to around 30A are commonly used in applications such as caravans, RVs (recreational vehicles), boats, and small buildings like cabins or tiny homes. These charge controllers are often designed to accommodate the specific needs and constraints of these off-grid or mobile applications.

On the other hand, most larger, more advanced 40A+ MPPT solar charge controllers they are specifically designed for larger-scale off-grid power systems with solar arrays and powerful off-grid inverters. including residential, commercial, and industrial installations.

What Factors Affect the Current and Voltage Output by The Solar Controller?

Solar Irradiance: Solar irradiance refers to the intensity of sunlight reaching the solar panels. Higher levels of solar irradiance result in greater power generation by the solar panels, leading to higher current and voltage output from the solar charge controller.

Temperature: Solar panel performance is negatively affected by high temperatures. As temperature increases, the efficiency of solar panels decreases, leading to a decrease in both current and voltage output. Solar charge controllers may incorporate temperature compensation mechanisms to adjust charging parameters based on temperature fluctuations.

Shading and Obstructions: Shading or obstructions on the solar panels can reduce the amount of sunlight reaching the panels, thereby reducing power generation. This can result in lower current and voltage output from the solar charge controller.

Solar Panel Characteristics: The characteristics of the solar panels themselves, such as their size, type, and efficiency, directly influence the current and voltage output. Higher-efficiency panels can produce more power for a given area of installation, leading to higher current and voltage output.

System Voltage: Solar charge controllers are typically designed to operate with specific system voltages, such as 12V, 24V, or 48V. The system voltage affects the charging parameters and determines the maximum voltage output of the controller.

Battery State of Charge: The state of charge of the battery bank connected to the solar charge controller also affects its output. During bulk charging, when the battery is discharged, the charge controller delivers maximum current to the batteries. As the battery approaches full charge, the charge controller reduces the current output to avoid overcharging.

Type of Charge Controller: The type of charge controller used, such as PWM (Pulse Width Modulation) or MPPT (Maximum Power Point Tracking), also influences the current and voltage output. MPPT controllers can optimize power output from the solar panels under varying environmental conditions, leading to higher current and voltage output compared to PWM controllers.

Overall, these factors interact to determine the current and voltage output of a solar charge controller, with variations depending on environmental conditions, system configuration, and the specific characteristics of the solar panels and batteries involved.

MPPT Vs PWM Solar Charge Controllers

There are two main types of solar charge controllers, PWM(Pulse Width Modulation) and MPPT(Maximum Power Point Tracking), with the latter being the primary focus of this article due to the increased charging efficiency, improved performance and other advantages explained below.

1. PWM solar charge controllers

As shown in the diagram, In a PWM system, when the solar panel generates power, the controller switches the panel's output to match the battery voltage. This means that if the battery is at a lower voltage (like 12V), the panel will also operate at this voltage, which might not be its most efficient operating point. Consequently, PWM controllers may not capture the maximum available power from the solar panel, leading to reduced efficiency.

PWM controllers don't necessarily "force" the solar panel to operate at the battery voltage, but they regulate the voltage output of the solar panel by rapidly switching it on and off.

Here's how it works:

When the solar panel generates power, the PWM controller rapidly switches the panel's output on and off. This creates pulses of current that effectively control the amount of power flowing to the battery.

The duty cycle of these pulses is adjusted to regulate the average voltage supplied to the battery. When the battery is at a lower voltage (like 12V), the PWM controller adjusts the duty cycle to match this voltage. This means that the average voltage supplied to the battery will be close to its voltage level.

However, because PWM controllers regulate voltage by essentially turning the panel on and off, they are not able to extract the maximum power from the solar panel. This is because the panel may be capable of generating higher voltages or currents under different conditions, but the PWM controller restricts its output to match the battery voltage.

2. MPPT solar charge controllers

As shown in the diagram, MPPT controllers enable the panel to operate at it’s optimum power (Vmp) voltage which is more efficient.

solar charge controller

MPPT (Maximum Power Point Tracking) controllers enable the solar panel to operate at its optimum power voltage (Vmp), which results in higher efficiency compared to PWM controllers. Here's why:

Tracking the Maximum Power Point (MPP): Solar panels have a specific voltage (Vmp) and current (Imp) at which they can produce the maximum power output. This point varies depending on factors like sunlight intensity, temperature, and shading. MPPT controllers continuously track this optimal operating point by adjusting the voltage and current to match the conditions in real-time.

Efficient Energy Conversion: By operating the solar panel at its optimum power point, MPPT controllers ensure that the maximum amount of power is extracted from the panel under varying environmental conditions. This allows for more efficient energy conversion compared to PWM controllers, which typically regulate the panel voltage to match the battery voltage.

Dynamic Adjustment: MPPT controllers dynamically adjust the operating point of the solar panel to account for changes in sunlight intensity and other environmental factors. This flexibility optimizes energy harvesting even when conditions are not ideal, such as partial shading or varying temperatures.

Higher Overall Efficiency: The ability of MPPT controllers to maximize power output from the solar panel results in higher overall system efficiency. This means more energy is harvested from the same solar panel compared to PWM controllers, leading to better performance and increased energy production over time.

In summary, MPPT controllers enable solar panels to operate at their optimum power voltage (Vmp), which maximizes energy harvesting efficiency by continuously tracking the maximum power point under changing environmental conditions. This dynamic optimization leads to higher overall system efficiency compared to PWM controllers.

What is an MPPT or Maximum Power Point Tracker?

A maximum power point tracker, or MPPT, is basically an efficient DC-to-DC converter used to maximise the power output of a solar system.
The first MPPT was invented by a small Australian company called AERL way back in 1985, and this technology is now used in virtually all grid-connect solar inverters and all MPPT solar charge controllers.

The functioning principle of an MPPT solar charge controller is relatively simple.
Due to the varying amount of sunlight (irradiance) landing on a solar panel throughout the day, the panel voltage and current continuously vary.
To generate the most power, an MPPT sweeps through the panel voltage to find the sweet spot or the best combination of voltage and current to produce the maximum power.
The MPPT continually tracks and adjusts the PV voltage to generate the most power, no matter what time of day or weather conditions.
Using this clever technology, the operating efficiency greatly increases, and the energy generated can be up to 30% more than a PWM charge controller, Image cited in

PWM Vs MPPT Example

 Let's consider a hypothetical scenario with a solar panel rated at 100 watts and a 12-volt battery. We'll compare the performance of a PWM controller versus an MPPT controller under different conditions.


  • Solar panel rated power (Wp): 100 watts
  • Battery voltage: 12 volts
  • Maximum Power Voltage (Vmp) of the solar panel: 18 volts
  • Maximum Power Current (Imp) of the solar panel: 5.56 amps

1. Sunny Day:

  • Sunlight intensity: High
  • Temperature: Moderate
  • No shading

PWM Controller:

  • Operates the solar panel at the battery voltage (12 volts)
  • Output power = Voltage * Current = 12V * 5.56A = 66.72 watts

MPPT Controller:

  • Tracks the maximum power point (Vmp = 18V)
  • Output power = Voltage * Current = 18V * 5.56A = 100 watts (maximizing power output)

2. Partial Shading:

  • Sunlight intensity: Partial shading on part of the solar panel
  • Temperature: Moderate

PWM Controller:

  • Operates the solar panel at the battery voltage (12 volts), possibly reducing further due to shading
  • Output power decreases due to shading, let's say to 50 watts

MPPT Controller:

  • Adjusts the operating point to mitigate shading effects, possibly by reducing the voltage to maintain higher current
  • Output power is maintained closer to the maximum, let's say at 90 watts

3. Low Light Conditions:

  • Sunlight intensity: Low (e.g., cloudy day or early morning/late afternoon)
  • Temperature: Moderate

PWM Controller:

  • Operates the solar panel at the battery voltage (12 volts)
  • Output power decreases significantly due to lower sunlight intensity, let's say to 30 watts

MPPT Controller:

  • Tracks the maximum power point even under low light conditions
  • Output power is maintained relatively higher, let's say at 60 watts

In this example, you can see that the MPPT controller consistently outperforms the PWM controller by maximizing power output under different conditions, resulting in higher overall energy harvesting efficiency.

How to Choose a Solar Charge Controller that Matches Batteries

Unlike battery inverters, most MPPT solar charge controllers can be used with various battery voltages from 12V to 48V.

For example, most smaller 10A to 30A charge controllers can charge either a 12V or 24V battery, while most larger capacity or higher input voltage charge controllers are designed for 24V or 48V battery systems. A select few, such as the Bateria Power -Sunrock 150V/250V range, can be used on all battery voltages from 12V to 48V. Besides the current (A) rating, the battery voltage also limits the maximum solar array size connected to a solar charge controller.

Several factors need to be considered to ensure optimal performance, safety, and longevity of the battery system:

Voltage Compatibility: Lithium batteries have different voltage characteristics compared to lead-acid batteries. Ensure that the solar charge controller is compatible with the voltage range of the lithium battery bank. Lithium batteries typically operate at higher voltages per cell (e.g., 3.2V to 3.7V for LiFePO4), so the charge controller should be able to handle these voltages.

Charge Algorithm: Lithium batteries require specific charging algorithms to prevent overcharging, undercharging, and overheating. Look for a solar charge controller that supports lithium battery charging profiles, such as CC/CV (Constant Current/Constant Voltage) charging with a voltage limit and a current taper-off stage.

Temperature Compensation: Lithium battery charging is sensitive to temperature variations. Choose a charge controller with temperature compensation features to adjust charging parameters based on ambient temperature changes. This helps optimize charging efficiency and battery lifespan.

Battery Protection Features: Ensure that the charge controller includes built-in protection mechanisms to prevent overcharging, over-discharging, short circuits, and reverse polarity. These safety features are crucial for protecting the lithium battery and ensuring reliable operation.

Communication and Monitoring: Some advanced charge controllers offer communication interfaces (such as RS485, RS232, or Bluetooth) and monitoring capabilities to provide real-time data on battery status, charging parameters, and system performance. This allows for better control, monitoring, and troubleshooting of the solar power system.

Efficiency and MPPT: Consider using an MPPT (Maximum Power Point Tracking) charge controller, which maximizes the power output from the solar panels by dynamically adjusting the operating point to match the battery's voltage and charge status. MPPT controllers are more efficient and can harvest more energy compared to PWM controllers, especially in variable weather conditions.

Compatibility with Solar Panel Array: Ensure that the charge controller's maximum input voltage and current ratings are suitable for the solar panel array connected to it. The controller should be able to handle the maximum voltage and current produced by the solar panels without exceeding its limits.

Overall, supporting a wide range of battery voltages makes MPPT solar charge controllers more versatile, adaptable, and appealing to users with diverse solar power system requirements. It simplifies system design, installation, and maintenance while offering flexibility for future expansion or changes in battery configurations.

How to Calculate What Size Solar Charge Controllers I need

To calculate the size of the solar charge controller needed for your solar power system, you'll need to consider several key factors:

Solar Panel Array Voltage and Current: Determine the total voltage and current output of your solar panel array. This information is typically provided by the solar panel manufacturer and depends on the number of panels and their specifications (e.g., wattage, voltage, current).

Battery Bank Voltage: Determine the voltage of your battery bank. Common battery voltages for off-grid solar systems include 12V, 24V, and 48V.

System Voltage: Decide on the system voltage, which should match the voltage of your battery bank. This will determine the nominal voltage of the charge controller.

Maximum Power Point Tracking (MPPT) vs. Pulse Width Modulation (PWM): Decide whether you want to use an MPPT or PWM charge controller. MPPT controllers are more efficient and can handle higher voltage solar panel arrays, making them suitable for larger systems or systems with varying weather conditions.

Current Capacity: Determine the maximum current rating (in amps) of the charge controller. This should be sufficient to handle the maximum current output of your solar panel array.

Oversizing: It's generally recommended to oversize the charge controller by 25-30% to accommodate any future expansions or inefficiencies in the system.

Once you have gathered this information, you can use the following formula to calculate the size of the solar charge controller:

Controller Current Rating (in Amps)=Total Solar Panel Array Power (in Watts)/​(Battery Bank Voltage*controller efficiency)

Here's a breakdown of the steps involved:

  1. Calculate the total solar panel array power by multiplying the total wattage of your solar panels by the efficiency factor. For example, if you have five 100-watt solar panels with an efficiency factor of 0.85 (85%), the total solar panel array power would be 5×100 W×0.85=425 W.

  2. Determine the battery bank voltage (e.g., 12V, 24V, 48V).

  3. Choose the system voltage, which should match the battery bank voltage.

  4. Determine the controller efficiency. This is typically provided by the manufacturer and represents the efficiency of the charge controller in converting solar energy into usable electricity.

  5. Plug the values into the formula to calculate the current rating (in amps) of the solar charge controller needed for your system.

Keep in mind that this formula provides an estimate, and actual controller sizing may vary depending on specific system requirements, manufacturer recommendations, and other factors. Additionally, it's essential to consult with a qualified solar energy professional to ensure the proper sizing and compatibility of the charge controller with your solar power system.

1.Basic sizing guide

As a general rule for lead-acid batteries, the charge controller Amp (A) rating should be 10 to 20% of the battery Amp/hour (Ah) rating.

For Lithium batteries, the controller Amp (A) rating can be higher, typically from 20% up to 50% of the battery Amp/hour (Ah) rating.
Always remember to check the battery manufacturer's specifications and never exceed the maximum charge rating!

For example, a 100Ah 12V lead-acid battery will need a 10A to 20A solar charge controller.
During sunny weather, a 150W to 200W solar panel should generate the minimum 10A* charge current needed for a 100Ah battery to reach the adsorption charge voltage, provided it is orientated correctly and not shaded. Always refer to the battery manufacturer’s specifications.

2.Advanced Guide to off-grid solar system design

Before selecting an MPPT solar charge controller and purchasing panels, batteries or inverters, you should understand the basics of sizing an off-grid solar power system. The general steps are as follows:

Calculate the loads - Use a load table to estimate the amount of energy used per day (kWh)
Battery capacity - based on the loads, determine the battery capacity needed in Ah or kWh
Solar size - Calculate how many solar panel/s you need to charge the battery (W)
Charge controller size - Select a suitable MPPT Solar Charge Controller to match the solar (A)
Inverter Size - Select an off-grid inverter to suit the continuous and peak loads.

3.Estimate the loads

The first step is to determine the loads using a load calculator

This estimates how much energy will be required pe day based on what appliances you will be using and for how long. This is done by multiplying the appliance power rating (W) by the average runtime (hr) to give you the total Wh. Alternatively, use the average current draw (A) multiplied by the runtime (hr) to calculate the energy in Ah.

In temperate climates with shorter winter days and high loads, the load table should be focused more on the winter load demand, as it will be generally higher.

Energy required in Watt-hours (Wh) = Power (W) x Time (hrs)
Energy required in Amp-hours (Ah) = Amps (A) x Time (hrs)

Once this is calculated for each appliance or device, the total daily energy requirement (kWh) can be determined.

4.Sizing the Battery

The total load measured in Ah or Wh load is used to size the battery.
Lead-acid batteries are sized in Ah while lithium batteries are sized in either Wh or Ah.

The allowable daily depth of discharge (DOD) is very different for lead-acid and lithium, see more details about lead-acid Vs lithium batteries. In general, lead-acid batteries should not be discharged below 70% SoC (State of Charge) on a daily basis, while Lithium (LFP) batteries can be discharged down to 20% SoC on a daily basis. Note: Lead-acid (AGM or GEL) batteries can be deeply discharged, but this will severely reduce the life of the battery if done regularly.

For example: If you have a 30Ah daily load, you will need a minimum 100Ah lead-acid battery or a 40Ah lithium battery. However, taking into account poor weather, you will generally require at least two days of autonomy - so this equates to a 200Ah lead-acid battery or an 80Ah lithium. Depending on your application, location, and time of year, you may even require 3 or 4 days of autonomy.

 5.Sizing the Solar

The solar size (W) should be large enough to fully charge the battery on a typical sunny day in your location.There are many variables to consider including panel orientation, time of year & shading issues.
This is actually quite complex, but one way to simplify things it to roughly work out how many watts are required to produce 20% of the battery capacity in Amps.

Solar sizing Example:
Based on the 20% rule, A 12V, 200Ah battery will need up to 40Amps of charge. If we are using a common 250W solar panel, then we can do a basic voltage and current conversion:
Using the equation (P/V = I) then 250W / 12V battery = 20.8A

In this case, to achieve a 40A charge, we would need at least 2 x 250W panels. Remember there are several loss factors to take into account, so slightly oversizing the solar is a common practice - See more about oversizing solar below.

6.Solar Charge controller Sizing (A)

The MPPT solar charge controller size should be roughly matched to the solar size. A simple way to work this out is using the power formula:
Power (W) = Voltage x Current or (P = V*I)

If we know the total solar power in watts (W) and the battery voltage (V), then to work out the maximum current (I) in Amps we re-arrange this to work out the current - so we use the rearranged formula:
Current (A) = Power (W) / Voltage or (I = P/V)

For example:
if we have 2 x 200W solar panels and a 12V battery, then the maximum current = 400W/12V = 33Amps. In this example, we could use either a 30A or 35A MPPT solar charge controller.


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