The PWM controller is in essence a switch that connects a solar array to a battery. The result is that the voltage of the array will be pulled down to near that of the battery.
The MPPT controller is more sophisticated (and more expensive): it will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement, of the battery plus load. Thus, it essentially decouples the array and battery voltages so that there can be, for example, a 12 volt battery on one side of the MPPT charge controller and a large number of cells wired in series to produce 36 volts on the other.
The MPPT controller are most effective under these circumstances:
Cool, and/or cloudy or hazy days – when the extra power is needed the most.
- Cold weather – solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in spring/fall seasons – the time when sun hours are low and you need the power to recharge batteries the most.
- Low battery charge – the lower the state of charge in your battery, the more current a MPPT puts into them – another time when the extra power is needed the most. You can have both of these conditions at the same time.
- Long wire runs – If you are charging a 12 volt battery, and your panels are 50 feet away, the voltage drop and power loss can be considerable unless you use very large wire. That can be very expensive. But if you have four 12 volt panels wired in series for 48 volts, the power loss is much less, and the controller will convert that high voltage to 12 volts at the battery. That also means that if you have a high voltage panel setup feeding the controller, you can use much smaller wire.