What is the difference between PWM and MPPT charge controllers?

Also: It seems that PWM charge controllers are much less expensive - what's the difference?

These questions get asked a lot. So I'm posting them to start a general discussion and to link to some great explanations from the Internet at large.


The shortest, most meaningful answer (that I can offer) is that almost all controllers use Pulse Width Modulation (PWM) to vary the voltage produced by a solar panel for the various stages of battery charging (Float, Bulk Charge, Trickle, Absobtion are a few). Maximum Power Point Tracking (MPPT) chargers add some logic (and expense) to hold the solar module as close to its ideal voltage (VMP) and the battery at its ideal current (IMP). So, an MPPT charger allows a wider range of modules and temperature / irradiance to charge a particular battery voltage (12,24,36, etc.).

Now, here are two excellent discussions on the subject: