PWM stands for Pulse Width Modulation. The devices can be explained as an electrical switch between batteries. It can be quickly switched on and off. Which allows the desired voltage to be obtained to charge the batteries.
So what is the difference?
MPPT charge control devices are more efficient. The current is drawn out of the panel at the panel.
Where as a PWM device, the current is drawn out of the panel at just above the battery voltage. This is not to say that PWM devices do not have place in a system set up.
Choosing the right solar controller/regulator.
PWM devices are a good low cost option for small systems where the efficiency of the system is not critical such as trickle charging. Also it’s good for panels with a maximum power voltage of up to 18V for charging a 12v battery.
MPPT controllers are best for larger systems where additional energy harvesting is worth while and the solar array voltage is substantially higher than the battery voltage. An example of this is using house solar panels to charge 12v batteries.
