A solar charge controller, also known as a charge regulator, plays a crucial role in managing the charging and discharging of your battery bank. Its primary function is to prevent overcharging and over-discharging of your batteries, which can significantly extend battery lifespan and ensure a reliable power supply in your off-grid system.

Choosing the right size solar charge controller is a critical step in designing an efficient and reliable off-grid solar power system. By “size” we are referring to the charging current and voltage capacity of the controller.  The size of the solar charge controller you need depends on several factors: 

Solar Array Capacity

This is typically measured in watts (W) or kilowatts (kW) which is a product of the amperage and voltage produced by the array. To determine the size of the solar charge controller, you’ll need to know the total capacity of your solar array. For example, if you have a solar array that delivers a maximum of 20 Amps and a maximum of 100 Volts to your controller you will need a charge controller that specifies the ability to receive these currents and voltages on its datasheet or operation manual. And you will need to compensate for temperature effects.

Battery Bank Voltage

The voltage of your battery bank is another crucial factor. Most off-grid systems use 12-volt, 24-volt, or 48-volt battery banks. Your charge controller needs to be compatible with the voltage of your battery bank. Make sure to match the voltage ratings, because incompatible voltages can cause operation failure with your controller.

Daily Power Consumption of Your Loads

This will help you understand how much energy your solar panels need to generate and how quickly your batteries need to be recharged. Consider all the electrical loads (lights, fans, radio/monitoring equipment, and other devices/appliances you plan to run in your off-grid system and their energy requirements. This information will guide you in sizing your solar array and ultimately your charge controller.

Temperature Considerations

The temperature at your location can affect the performance of your solar panels and, in turn, the charge controller. Most charge controllers are rated based on a specific ambient temperature, often around 25°C (77°F). If you live in an area with extreme temperatures, make sure you have temperature compensation features with your charge controller, which can adjust charging parameters to optimize battery performance in varying temperature conditions.  It is important to calculate your solar array’s voltage output at the lowest recorded temperature where your system is located since solar array voltage output increases at low temperatures.  This voltage must never exceed your controller’s open circuit voltage (Voc), otherwise you may destroy your controller and create a fire hazard.

Efficiency of the Charge Controller

The efficiency of the charge controller is another factor to consider. Charge controllers are not 100% efficient, and some power is lost as heat during the charging process. To compensate for this, you may need to oversize your charge controller slightly. For example, if your solar panel array generates 2 kW, you might choose a 2.5 kW charge controller to ensure that enough power is delivered to your batteries when you factor in inefficiencies, intermittent clouds and low sun angles of incidence.

Future Expansion

Consider your future plans for expanding your off-grid system. If you anticipate adding more solar panels or increasing your battery bank’s capacity, it’s a good idea to choose a charge controller with some room for growth. This will save you the hassle and cost of replacing the controller when you decide to expand your system.

Controller Type

There are two main types of solar charge controllers.

  1. PWM (Pulse-Width Modulation) controllers:
    – Operate at battery voltage which is generally below the maximum power voltage (Vmp)
    – Are suitable for small module configurations
    – Are often chosen for very hot climate which will not yield as much MPPT boost
    – Are less expensive than MPPT controllers
  2. MPPT (Maximum Power Point Tracking)  controllers:
    – Convert excess input voltage into amperage
    – Operate at maximum power voltage (Vmp)
    – Are suitable for large module configurations that have a lower cost per watt
    – Provide more boost than PWM, especially during cold days and/or when the battery voltage is low

MPPT controllers use newer technology that supports more module types (e.g. 30 and 60 cells in addition to 72 cells), and they support array oversizing.  Your choice between MPPT and PWM controllers depends on your budget and specific system requirements.

What size charge controller is needed for a 1200 Watt solar array?

There is no way to know what size controller is appropriate for a 1200 watt solar array unless the current and voltage specs of the array are known for the lowest and highest operating temperatures that the array will be exposed to.  Since Watts = Amps X Volts, a 1200 Watt array could be producing approximately 36 Volts and 33.33 Amps, or 18 Volts and 66.67 Amps, or some other combination of Amps and Volts that produces 1200 Watts at standard test conditions (25 degrees C).

If the specifications for the array do in fact indicate that the array delivers 36 Volts at 33.33 Amps at 25 degrees C, then we would know that the controller would need to be rated for higher than those voltage and amperage limits in order to compensate for temperature effects and safety factors.  Morningstar offers controllers with Voc limits of 30V, 60V, and other higher limits.  For this example, a Morningstar controller rated for at least 60V and 40 amps would most probably suffice for the array, but before choosing the controller you would also need to factor in load considerations, battery size, and battery chemistry.

To ensure proper charge controller selection and the long-term performance and reliability of your off-grid system, it’s helpful to consult with a knowledgeable solar installer or engineer who can help you make the best choice based on your specific needs and circumstances. Some solar distributors can size your solar charge controller and your entire off-grid system for that matter. If you need help determining the right controller for your application, you can contact us and we and one of our distribution partners can help determine this for you.  Our string calculator can also help you know how well a specific Morningstar controller will meet your system requirements.