What Size Charge Controller You Need (Calculated)

There are two types of charge controllers available in the market. Depending on the number and power of the solar panels to be paired with the number and voltage of the battery bank, a selection of the best size charge controller can be made. Charge controllers are rated according to amperage.

Charge controllers are sized to cope with the input voltage and current from the solar panels and how this power is most efficiently transferred to the battery bank. A safety factor of 25% is added to the solar array amperage to compensate for environmental factors.

Additional factors to consider when selecting the type and size of a charge controller are:

  • Budget
  • Design lifespan of the system
  • Climate conditions (cold temperatures, marine)
  • How many solar panels do you have to meet your energy needs
  • The number, size, and type of batteries in your battery bank

Let’s look at selecting the correct type and size charge controller for your system?

Solar Panel Charge Controller With A Hand On
Solar Panel Charge Controller With A Hand On

What Type And Size Charge Controller To Select

1. Pulse Width Modulation (PWM) charge controllers

For solar systems where the output voltage of the solar panels must match the input voltage of the battery bank, the Pulse Width Modulation (PWM) charge controllers are ideal.

They are less expensive and ideal for smaller simple solar systems for recreational vehicles, tiny homes, or vans.

The PWM charge controller charges the battery bank with short current pulses at the same charge voltage as the solar panel output voltage. PWM charge controllers are unable to limit their current output.

Suppose the solar panel array has 30A (amp) output current. In that case, the charge controller selected will have to cope with a minimum of 30 A. To compensate for solar array performance in cold weather when the panels operate more efficiently, a safety factor of 25% must be added to the 30 A output current.

The PWM charge controller size must be 30 A x 1.25 = 37.5 A for such a system. We need to consider both the amperage and the voltage when matching the correct size charge controller to the system.

Ideal For Simple Systems

PWM charge controllers are available in 10 A, 20 A, and 30 A capacities and are ideally suited for simple systems to charge 12 V and 24 V battery banks. A 10A PWM charge controller can support a 120 W solar array to charge a 12 V battery bank (120W/12V = 10A) or it can support a 240 W solar array to charge a 24 V battery bank (240W/24V = 10A).

For a 240W 12 V solar array to charge a 12V battery bank (240W/12V = 20A) a 20 amp PWM Charge controller is required. It is imperative that the voltage of the solar array matches the charge voltage of the battery bank with PWM-type controllers.

PWM controllers are not as complex or expensive as MPPT controllers. They work best in small PV systems where operational efficiency is not critical. They are best for sunny and warm conditions and have a long lifespan due to the simplicity of the design.

On the negative side, PMW controllers are less efficient and can only be deployed in systems where the solar array and the battery bank have matching voltage requirements.

2. Maximum Power Point Tracking Controllers (MPPT)

MPPT charge controllers are best suited for large solar arrays and battery banks in domestic off-grid or marine applications where solar power is one source of power input only. These controllers are highly efficient and best for getting the best power conversion from solar to a battery.

The MPPT controller will convert the maximum power voltage from the solar array to the voltage required to optimally charge the battery bank. The voltage of the solar array does not have to match the battery bank’s voltage.

Automatic Adjustment

An MPPT controller will monitor the state of charge of the battery bank and adjust the charge voltage and current accordingly, protecting the battery chemistry and longevity of the battery bank.

In cloudy conditions, the MPPT will decrease the amount of charge current while maintaining the optimal charge voltage to the battery bank. As the clouds disappear and the stronger sunlight increases the output, the MPPT will increase the charge current but maintain the optimal charge voltage.

MPPT charge controllers can be sized to suit the size of the solar array and the voltage requirements to charge the battery bank optimally.

Select Correct MPPT Amp

Large solar arrays can generate power, but the MPPT controller will limit the output. It would be inefficient to have panels delivering 80 A of current to an MPPT controller with a 40 A output current rating.

In this example, it would be better to have two 40 A MPPT controllers controlling the 80 A input current from the solar panels. The input voltage rating of the solar array can be much higher than the charge voltage requirement of the batteries.

MPPT Voltage

If the MPPT charge controller is rated to accept 100 V input from the solar array, it would need to step this voltage down to 12V or 24V depending on the battery system voltage.

Example: If we have 4 x 100 W panels in series at 5 A, each panel will have an open-circuit voltage of 22.5 V. The 4 panels will each deliver 22.5 V for a total of 90 V, which is within the 100 V rating for the controller.

MPPT charge controllers are highly efficient and ideal for more complex solar systems where energy efficiency is critical. These controllers are well suited for cold climates and to govern the input voltage from a large array of solar panels.

MPPT controllers are significantly more expensive than PWM controllers and do not have long life cycles due to the component complexity.

Typical MPPT Amperage Sizes

  • A 20A MPPT charge controller can support up to 260W input for a 12V battery bank. (260W / 20A = 13 A) or 520W solar input for a 24V battery bank. (520W / 24V = 21.67 A)
  • A 30A MPPT charge controller can support up to 400W solar input for a 12V battery bank. (400W / 12V = 33.33 A) or 800W solar input on a 24V battery bank. (800W / 24V = 33.33A)
  • A 40A MPPT charge controller can support 520W solar input for a 12V battery bank. (520W / 12V = 43.33A) or 1040W for a 24V battery bank. (1040W / 24V = 43.33A)
  • A 60A MPPT charge controller can support 800W on 12V (66.67A), 1600W on a 24V (66.67A), 2400W on 36V (66.67A) or 3200W on 48V battery bank (66.67A)
  • A 100A MPPT charge controller can support 1300W solar input for a 12V battery bank (108.33A) or 2600W on a 24V battery bank (108.33A) or 3900W on a 36V battery bank or 5200W on a 48V battery bank (108.33A).


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