Charge controllers play a key part of every solar installation. The solar charge controller sits between the solar panels and battery bank. Both MPPT and PWM charge controllers limit the amount and rate of charge to your batteries, provide overload protection, disconnect at low voltages, and block reverse current. You’ll typically need a charge controller for any solar panel larger than five watts.

PWM charge controllers are the cheapest charge controller option, best for warm sunny weather, and performs best when the battery is near the full state of charge. They are ideal for small scale applications because the solar panel system and batteries have to have matching voltages. PWM controllers regulate the flow of energy to the battery by reducing the current gradually, called "pulse width modulation". When batteries are full, a PWM charge controller will supply a tiny amount of power to keep batteries full.

Maximum Power Point Tracking charge controllers are suitable for situations where the solar array voltage is higher than the battery voltage, highly space and energy efficient, ideal in larger systems where the additional energy production is valuable, best in colder, cloudier environments, and performs best when the battery is in a low state of charge. MPPT controllers will actively monitor and adjust their input to regulate a solar system’s current, and they will step down the voltage and boost the current. 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 again.

Charge controllers are sized based on the solar array's current and the solar system’s voltage. To size your system, we recommend using the Renogy solar calculator. 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. If your solar system's volts were 12 and your amps were 14, you would need a charge controller that had at least 14 amps. However due to environmental factors, you need to factor in an additional 25%. This brings the minimum amps that this charger controller must have to 17.5 amps. In this example, you would need a 12 volt, 20 amp charge controller.