Why an MPPT Charge Controller is Needed for Solar Panels in Certain Situations
Most higher wattage solar panels are designed for the grid-tie market where array voltage is not as critical as compared to an off-grid battery based system. A 200+ watt panel with an operating voltage in the range of 28 to 30 volts will not properly charge a 24 volt battery bank that is is intended to be bulk charged to 29.6 volts, or even equalized to 31 volts. Warm weather or weak sunlight will keep the panel from reaching full voltage, thus keeping the battery bank from ever reaching a 100% fully charged state. Using such a panel with a traditional PWM controller to charge a 12 volt battery bank would lose half the available power rendering the purchase of such a panel moot in the first place.
Using a Maximum Power Point Tracking Charge Controller for Solar Panels
So how can we make use of such panels in off-grid battery based systems? The answer is to use a Maximum Power Point Tracking charge controller and wire the array at a higher voltage than the battery bank.
The original intended advantage of an MPPT charge controller is that they yield 5-25% more power, depending on weather and temperature, harvested from a given array compared to a traditional PWM controller. A secondary effect that we can take advantage of is the voltage regulation. An MPPT controller doesn't care what the incoming array voltage is (within some limits). It simply wants to know what the output voltage to the battery bank should be.
Thus if an MPPT controller is used, a single panel at 29ish volts can properly charge a 12 volt battery bank, getting full usage of the wattage available. Two panels can be wired in series at 58 volts to charge a 24 volt battery bank (or a 12 volt bank), and at least 3 panels in a series string at 87 volts would be needed for a 48 volt battery bank.
The other advantage of an MPPT controller is that having the array wired at a higher voltage minimizes the amperage that has to be transmitted from the array to the charge controller, allowing smaller gauge wire to be used. The savings in copper wire alone often can justify the higher cost of the MPPT controller.
To summarize, MPPT controllers let us utilize the larger wattage solar modules (that tend to have better price points than small modules), they harvest more power over the life of the system, and can offer a cost savings in the transmission wire needed from the array.