A PWM Controller is, in essence, like anelectric switch between the solar panels and the battery. They are effective inslowly lowering the amount of power going to your batteries as they get closerto fully charged, thus keeping them at a fully charged state while eliminatingthe stress on your batteries. In addition to the above, an MPPT Controller isalso capable of converting excess voltage to amperage. The benefit of thisfeature is it keeps an optimal charge voltage while requiring less time tofully charge your batteries. It also allows for reduced amounts of power lossas the conversion of excess voltage to amps allows for greater voltage totravel through the panel to the controller.