Understanding Charge Controllers

Charge controllers come in many different shapes and sizes. Greenwired offers a range of controllers, from "simple" pulse width modulation (PWM) units that are designed to keep your batteries from overcharging, all the way up to the high tech Maximum Power Point Tracking (MPPT) units that modulate solar array voltage to maximize power output as well as provide automated functions for your battery bank.

When are charge controllers needed?

Any time you have a solar module (panel) hooked up to a battery bank for charging a battery you will need to prevent the battery from overcharging. To do this a charge controller is needed.

If your solar module is connecting solar directly to a load (fan, or pump) or to the grid, a charge controller is not needed.

What is a "simple" Pulse Width Modulation (PWM) charge controller, and when do you need it?

A PWM controller watches your battery for when it reaches a near charge setpoint, then slowly reduces the charging current to avoid overcharging the battery.


If you can answer YES to all of the following questions, than a PWM controller may be right for you. If not, look at the MPPT charge controllers below.

  • Is your solar array designed to charge one battery or a bank of batteries?
  • Is the distance from solar modules to batteries less than 50 ft (unless wire size is large enough to prevent voltage drop)?
  • Is the total solar array wattage less than ~450 Watts?
  • Is the panel voltage (or array voltage) correct for battery bank? (See table below.)
Nominal Battery Voltage Suggested Vmp for solar module
12 17 to 21
24 34 to 38
48 64 to 68


What is a Maximum Power Point Tracking (MPPT) charge controller, and when do you need it?

MPPT charge controllers provide a method of capturing and using the most power from your PV solar modules (panels). They are designed to convert the module’s maximum power voltage down to battery voltage, allowing the solar module to operate at its peak efficiency.

Other benefits include using  strings (groups)  of modules to prevent or increase voltage, decrease amperage, and prevent voltage drop in transmitting the power.  (The word “tracking" has nothing to do with mechanically moving the PV modules to track the sun.)

MPPT controllers have become the industry standard for off grid power systems.  If you answer YES to any of these questions then an MPPT controller is right for you.

  • Will your solar array be designed to charge a battery or a bank of batteries?
  • Is the distance from solar modules to batteries more than 50 ft?
  • Is the total solar array wattage more than ~450 Watts?


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