PWM Charge Controller Sizing

Solar panels that are the same “nominal” voltage of your battery bank use PWM style charge controllers since the input voltage is close enough to that of your battery bank to allow proper charging without overcharging the cells. The short method for sizing a PWM style charge controller is to take your total wattage of your array and divide that number by the nominal voltage of your battery bank. Using OHM’S law, we can use this method to give us a rough idea of the size charge controller needed. Always consult the manufacturer data sheet to confirm your calculations.

12V Example:
140 watts of solar / 12V battery = 11.66amps
Add 20% for safety margin to account for the short circuit current
11.66 amps x 1.25 = 14.57 amps
Round up to the nearest charge controller size that is the same voltage as your system.
A good fit would be the Morningstar SunSaver SS-20L-12V charge controller.

24V Example:
300 watts of solar / 24V battery = 12.5 amps
Add 125% for safety margin to account for the short circuit current
12.5 amps x 1.25 = 15.6 amps
Round up to the nearest charge controller in the same voltage as your system.

You could use the SunSaver, SS-20L-24V or even the PS-30M. Any charge controller that is rated for at least 20 amps at 24V will work. Often times there is more than one option that will work and you can decide further if you want to include an option display/ meter or if you want a more basic unit for your system.

Several charge controller manufacturers are rated for 100% rating at continuous use, such as the Morningstar line and using the 125% over-current protection factor will keep you within the manufacturer electrical specifications. If you are not sure that your charge controller is rated for 100% continuous use, it is recommended that you estimate for another derate of 125% which will error on the side of caution.

MPPT CHARGE CONTROLLER SIZING

MPPT charge controllers are able to take a higher input voltage from your solar array to the lower voltage of your battery bank without overcharging your batteries. This is the more advanced style charge controller as they will sweep the voltage curve throughout the day to maximize your energy harvest resulting in about 10 – 30% (on average) more power production from your solar array. Sizing mppt charge controllers can be more involved as there are several considerations to take into account. There are sizing tools available as well as technical support representatives who can assist you.

MPPT Solar Charge Controller Sizing Tools from the Manufacturers

• Morningstar Sizing Tools
• OutBack Power String Sizing Tool
• Midnite Solar String Sizing Tool
• Blue Sky Energy Sizing Tools

You will get the most out of your system if your solar panel input voltage is over twice that of your battery bank with an MPPT charge controller.

As a rule of thumb, most 150VDC rated charge controllers will accept a maximum of (3) 60 or 72 cell panels wired in series. For charge controller sizing, you will always use the open circuit voltage (VOC) and the short circuit current (ISC) to make sure that your system falls within the electrical guidelines of the charge controller manufacturer. Reference the manufacturer’s data sheet on the charge controller you are looking at to see their maximum input as well.

The OutBack FM80 Data sheet states the following:
NEC Recommended Solar Maximum Array STC Nameplate:
12VDC systems: 1000W
24VDC systems: 2000W
48VDC systems: 4000W
60VDC systems: 5000W

Example:
Using a 60 cell 300 watt Canadian Solar panel: CS6K-300MS with the below electrical specifications:

12V battery system – (1) solar panels (32.5 VMP/ 39.7VOC)
24V battery system – (2) panels wire in series (65 VMP/ 79.4VOC)
48V battery system – (3) panels wired in series (105.75 VMP/ 119.10VOC)

For a 3600 watt PV array consisting of (12) 300 watt solar panels
(4) parallel strings of (3) panels wired in series:

Each string:
ISC (9.83) x (4) parallel strings = 39.32 x 1.20 = 47.184 amps
VOC (39.7) x 3 panels wired in series = 119.1 VOC
Both of these calculations fall into acceptable ratings to use the FM80 charge controller.
For colder environments you also want to account for the temperature coefficient for colder climates.

COLD CLIMATE

If you are installing in a location that is subject to very cold sunny days, then you will also need to take into account the temperature coefficient for the solar panels you are using. Some charge controller manufacturers refer to this a HVOC, Hyper Voltage Open Circuit rating. We will again reference the CS6K-300MS for example and an installation in the Sierra Mountains which are characterized by severe winters and heavy snowfall, with the average temperature of the coldest month < -3°C (26.6°F) and the average temperature warmest month > 10°C (50°F). These climates occur only in the higher areas of Sierra Nevada, typically above 6,000-7,000 ft.

Since the Canadian Solar 300 watt panel has a Temperature Coefficient (Voc) -0.29 % / °C at 25 degrees celsius, we need to account for the colder potential of -3 degrees celcius for the area we are installing.

(25) degrees solar panel rating – (-3 degrees) Sierra Mtns. = 28 degrees difference
You then multiply the temperature coefficient (-.29) x (28) = 8.12
You then add this to your string sizing calculation to make sure that you are not exceeding the manufacturer’s maximum voltage input rating.

HIGH VOLTAGE

If your array is a few hundred feet away from your battery, or if you are retrofitting an existing grid-tie system, you may want to consider the 600V charge controllers. These units are sized very similar to grid-tie inverters in the way that you can string your solar panels in series up to 600V. Some benefits are reduced wiring sizing and reduced voltage drop. For example, you could take (10) CS6K-300MS solar panels (39.7VOC) and wire them in series to get 397V. You would still want to account for the temperature coefficient should you be installing in a cold climate and also that your battery bank is a sufficient size to handle this size system. The Morningstar TS-MPPT-60-600V-48-DB would be a great option to consider for your high voltage system.

CONCLUSION

We hope this information has been helpful in your quest to size the proper charge controller for your PV system. This is meant to be a helpful guideline and system designs will vary.

For more information on overall system design you can reference our blog post, “How to start building your off-grid system” or for assistance in selecting your charge controller, please don’t hesitate to call us at 888-899-3509 ext 1.