How to Size a Solar Charge Controller

In Controls & Electronics by Kayla Jlassi

Introduction

It is important to have the solar charge controller correctly sized to properly and safely operate your solar power system. How you size your solar charge controller is dependent on the type of charge controller you have. (Refer to SunWize’s tech note on Solar Charge Controller Types for more information.) Due to popularity, we will focus on sizing MPPT Charge Controllers. Let's get started!

MPPT Charge Controllers

MPPT charge controllers currently limit their output, so technically you can make the solar array as large as you want and the charge controller will limit the output current to the maximum allowable. There is a practical limit though, if the solar array is too large then the power is just wasted, since the charge controller is always limiting the output. A typical recommendation is to limit the solar array to 110%-125% of the maximum controller rating.

Example: You have a 60A MPPT charge controller and a nominal 24VDC battery. That means we have 1440W of power. We then go ahead and calculate the 110%-125% of the maximum controller rating to get a range of 1584W  to 1800W. The complete calculation looks like this: 60A * 24V = 1440W * 1.1 or 1.25 = 1584W-1800W

Again, a larger solar array will not damage anything; it’s just a waste of energy as the controller will always be restricting the maximum output to the current limit. Overcurrent protection can be rated to the maximum current if the overcurrent device is designed to operate at 100% of rating. As an example, you can use a 60A circuit breaker with a 60A MPPT charge controller if the circuit breaker is rated to operate at 60A continuously. This is per an exception to the NEC. Article 690.8 (B)(1)(a) states “Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating.”
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All Other Charge Controllers

Shunt, series, and PWM charge controllers are unable to limit their current output. They simply pass through the array current. If the solar array can produce 40A of current and the controller is only rated to 30A, then the controller or system could be damaged. For this reason, the charge controller should be rated for the maximum array output plus a safety margin. Further NEC states that the array short circuit current (Isc) should be used, plus a 125% safety factor for cloud enhancement, plus an additional 125% safety factor for design margin.

Example: You have an array with an Isc of 8A.  Using the 125% safety factor, the charge controller should be rated for a minimum of 8A * 1.25 * 1.25 = 12.5A. 

Overcurrent protection should be sized the same way, and the next nearest size device used. In this example, a 15A circuit breaker would be used. Note: this should be paired with the maximum series string fuse rating of the solar module and a combiner box or additional overcurrent devices may be needed, refer to SunWize’s tech note on Combiner Box Requirements.

Some solid-state charge controllers may have inherent design margins or overload protections that allow you to use lower safety margins in your sizing.  If strict NEC adherence is not required, consult with the charge controller manufacture for their sizing recommendation.  Typically, it’s the array Isc * 110%-125%.  For safety purposes the overcurrent protection should still be sized per the NEC.

Conclusion

Understanding how to size a solar charge controller is critical to the functioning of your overall system. As we have seen, understanding the different charge controller types and appropriately sizing them can result in a properly running power system. Have more questions? Feel free to watch our more in-depth webinar on the Charge Controller Types and How to Size Them. Chat with us through our chat box or call us at (866) 827-6527.